Sentinel servers can be set to require auth like regular servers.

Explicitly didn't do `/toys` here because the versions aren't shared but if it makes sense: yeah let's add em. This structure is made for easier migration to https://github.com/NuGet/Home/wiki/Centrally-managing-NuGet-package-versions later (we should just need to remove the manual import in each project file (necessary for ordering):
```xml
<Import Project="$([MSBuild]::GetPathOfFileAbove('Directory.Packages.props', '$(MSBuildThisFileDirectory)../'))" />
```

If you're wondering why `Directory.Build.targets` doesn't work - I'm not sure. It's fine for `netcoreapp*` builds, but .NET Full framework throws a fit:
```
C:\git\StackExchange\StackExchange.Redis\tests\NRediSearch.Test\NRediSearch.Test.csproj : error NU1701: Package 'xunit 1.7.0.1540' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8' instead of the project target framework '.NETCoreApp,Version=v2.1'. This package may not be fully compatible with your project. [C:\git\StackExchange\StackExchange.Redis\Build.csproj]
C:\git\StackExchange\StackExchange.Redis\tests\BasicTestBaseline\BasicTestBaseline.csproj : error NU1604: Project dependency BenchmarkDotNet does not contain an inclusive lower bound. Include a lower bound in the dependency version to ensure consistent restore results. [C:\git\StackExchange\StackExchange.Redis\Build.csproj]
C:\git\StackExchange\StackExchange.Redis\tests\BasicTest\BasicTest.csproj : error NU1604: Project dependency BenchmarkDotNet does not contain an inclusive lower bound. Include a lower bound in the dependency version to ensure consistent restore results. [C:\git\StackExchange\StackExchange.Redis\Build.csproj]
C:\git\StackExchange\StackExchange.Redis\tests\BasicTest\BasicTest.csproj : error NU1701: Package 'BenchmarkDotNet 0.5.0' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8' instead of the project target
framework '.NETCoreApp,Version=v2.0'. This package may not be fully compatible with your project. [C:\git\StackExchange\StackExchange.Redis\Build.csproj]
C:\git\StackExchange\StackExchange.Redis\tests\BasicTestBaseline\BasicTestBaseline.csproj : error NU1701: Package 'BenchmarkDotNet 0.5.0' was restored using '.NETFramework,Version=v4.6.1, .NETFramework,Version=v4.6.2, .NETFramework,Version=v4.7, .NETFramework,Version=v4.7.1, .NETFramework,Version=v4.7.2, .NETFramework,Version=v4.8' instead of the project target framework '.NETCoreApp,Version=v2.0'. This package may not be fully compatible with your project. [C:\git\StackExchange\StackExchange.Redis\Build.csproj]
```
...but the approach in this PR is both works and is closer to future plans.

fix build to get green shields for Source Link and Deterministic

* Including SslProtocols in ConfigurationOptions.ToString()

* Removing an unnecessary method and making the code concise (based on code review comments)
Co-authored-by: Sampath Vuyyuru <sampath.vuyyuru@kincentric.com>

We were throwing in event handlers...that's not good. Also post-thread sleep in timer callbacks.

* fix naming of SentinelGetSentinelAddressesAsync

* tyop

* Sentinel: don't use async over sync with .Result
Co-authored-by: mgravell <marc.gravell@gmail.com>

* ConnectionMultiplexer: track global counts for deebugging

* Build dammit

* Move to per-multiplexer/add tests

Note: I know mutiplexer isn't spelled right - will fix that in a follow-up to avoid noise.

* Fix test key names

Broken since the 2.1 bump, oops

* Simplify the NoConnectionAvailable static

Simpifies usage for all callers. Also shares code and adds diagnostics to the "no connection" case.

* Add tests for NoConnectionException

* Failover: fix tests and debug some

SubscriptionsSurviveMasterSwitchAsync is a thorn in our side - moving to DEBUG.

* Remove bad check

Inner is irrelevant here - can be not-null depending on the connection race.

* Update message and add more tests!

* Bump pipelines to 2.1.6

Old message:
> To create a disconnected multiplexer, disable AbortOnConnectFail.

New message proposal:
> Error connecting right now. To allow this multiplexer to continue retrying until it's able to connect, use abortConnect=false in your connection string or AbortOnConnectFail=false; in your code.

This changes does the following:
- Moves the broadcast to be only before the master reconfiguration to both before *and* after - a fix following https://github.com/StackExchange/StackExchange.Redis/commit/88dcf0c989b25623fb8ffb1f4d48c24593246cfe to fix the gap.
- Tweaks tests by overall lessening runtime (and thus build server time). Overall, fixes a few static timeouts to be conditional (so they short circuit faster if met), and brings some tests that were some variant of the above into RELEASE since they're safe now.
- Changes `UntilCondition` to take a `TimeSpan`, just because clearer. Even though I IntelliSense completed `.FromMinutes()` earlier and watched it like an idiot for a while...I stand by this decision!
- Locks the `ITestOutputHelper` writer because...that was jacked up in races:

![off to the races](https://user-images.githubusercontent.com/454813/77232395-2f799980-6b77-11ea-8fb4-0398de25e313.png)

------

Note: a lot of the test changes are just optimizations to delays which allow longer but short-circuit sooner. The important changes are in broadcast and locking around the test runner. I can think of downsides to neither, but want some @mgravell eyes. This should resolve a lot of flaky-ness with local (and build agent) tests. Not all of it, but a lot of it!

Billing issues have cleared on the org - taking a stab at this.

First attempt: let's get a Linux CI going.

This bumps versioning for the 2.1.0 release, ready to GA!

This PR is derived from PR-692 and have been merged with the latest master commit.
Things that have been done:
1. review code for PR-692
2. fixed potential infinite loop in the code
3. Adapt code to success build with the latest master commit
4. Manual testing on 3 Sentinel nodes and 3 Redis nodes (connection and failover)

Usage:
```C#
                ConfigurationOptions sentinelConfig = new ConfigurationOptions();
                sentinelConfig.ServiceName = "mymaster";
                sentinelConfig.EndPoints.Add("192.168.99.102", 26379);
                sentinelConfig.EndPoints.Add("192.168.99.102", 26380);
                sentinelConfig.EndPoints.Add("192.168.99.102", 26381);
                sentinelConfig.TieBreaker = "";
                sentinelConfig.DefaultVersion = new Version(4, 0, 11);                 
                // its important to set the Sentinel commands supported
                sentinelConfig.CommandMap = CommandMap.Sentinel;

                // Get sentinel connection
                ConnectionMultiplexer sentinelConnection = ConnectionMultiplexer.Connect(sentinelConfig, Console.Out);
                // Create master service configuration
                ConfigurationOptions masterConfig = new ConfigurationOptions { ServiceName = "mymaster" };
                // Get master Redis connection
                var redisMasterConnection = sentinelConnection.GetSentinelMasterConnection(masterConfig);

                ...
               IDatabase db = redisMasterConnection.GetDatabase();                
               db.StringSet(key, value);
               ...
               string value1 = db.StringGet(key);

```

* re-implement GetEnumerator in EndPointCollection to be a bit more forgiving to concurrent changes during iteration

* poke release notes

Ensure that _activeMessage is always cleared, including if an exception is thrown while trying to send the message or flush the connection. Leaving _activeMessage set causes WriteMessageInsideLock to always return a "NoConnectionAvailable" error indefinitely (#1374)
Co-authored-by: Nick Craver <craver@stackoverflow.com>

Co-authored-by: Gunnar Liljas <gunnar.liljas@revide.se>

Asked for in #831 and low overhead so why not! I didn't do `RedisChannel` since it already has a constructor available. Tests added to ensure these are used in a way we don't break them.

Found while fixing up runs for #1067. Docker layer cache sucks :)

Adds support for running all Redis instances and configurations used in the tests inside of docker containers that work on all platforms. Just run `docker-compose up` inside of the `tests/RedisConfigs` folder.

In going through issues, I see many users confused by `Unspecified/` and it really serves no purpose. Let's get a quick win (and add a test for it).

Also, WSL2 it appears has clock jitter - we can accommodate that while having a valid test, fixing that local case.

Clarifies how IPv6 needs to be expressed in example usage.