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.

This is 2 of the 6 minutes of test CPU time locally....don't need to hang on this so much.

Tidying this up so the diff/fix in #1374 is easier to analyze.

Since logic exists in the getter, we need to set the field across for actual quality.  The `Issue883_Exhaustive` test illustrates the difference in the comparison (currently failing).

### NRediSearch
- Updated module so we're testing the right thing
- More output in the logging path for diagnosing issues
- Remove all the `FLUSHDB` calls masking issues
- Handle duplicate documents in latest module

Note: **locally** (on WSL 2), this fixes some issues and introduces stability but *does not* fix 2 scoring issues. Suggestions are coming back with scores > 1.0, which by the documentation is not supposed to happen...so I'm not sure what should be happening here. Here's the local WSL fail:
```
 NRediSearch.Test.ClientTests.ClientTest.TestGetSuggestionWithScore:
  Message: 
    StackExchange.Redis.RedisCommandException : Missing required fields:  score not within range: 2.11660146713257
  Stack Trace: 
    SuggestionBuilder.Build() line 99
    Client.GetSuggestionsWithPayloadAndScores(RedisResult[] results) line 1271
    Client.GetSuggestions(String prefix, SuggestionOptions options) line 796
    ClientTest.TestGetSuggestionWithScore() line 701
```
```
NRediSearch.Test.ClientTests.ClientTest.TestAddSuggestionGetSuggestionPayloadScores:
  Message: 
    StackExchange.Redis.RedisCommandException : Missing required fields:  score not within range: 2.71068739891052
  Stack Trace: 
    SuggestionBuilder.Build() line 99
    Client.GetSuggestionsWithPayloadAndScores(RedisResult[] results) line 1271
    Client.GetSuggestions(String prefix, SuggestionOptions options) line 796
    ClientTest.TestAddSuggestionGetSuggestionPayloadScores() line 642
```

However, the current fixes tidy up the CI pipeline so I recommend merging this as-is, then I'll keep following up with various local errors and I'm looking at some Docker test options as alternatives to WSL2 so that people can run either.

Fixes a break from #1254

Fixing a bug where READONLY is run before AUTH. Causes MOVED errors when reconnecting to Clustered Slaves. (#1367)

* Scoring in RediSearch

* Unit Test Added: EXPLAINSCORE
#1254