* Add netstandard2.0 target to SE.Redis projects

Two new conditional compilation constants are introduced:

* FEATURE_PERFCOUNTER - supported in net45 and net46
* FEATURE_THREADPOOL - supported in net45, net46, and netstandard2.0

* Add netcoreapp2.0 target framework to test projects

* Replace CORE_CLR conditional compilation symbol

with built-in NETSTANDARD1_5 or NETCOREAPP1_0 since now netstandard2.0
is also supported.

* Remove unnecessary conditional compilation constants

* Give defaultClientName a value when it is null
This prevents calling TryGetAzureRoleInstanceIdNoThrow() repeatedly in the null case.
* Upgrade xunit to 2.3.1
* Upgrade xunit to v2.4.0-beta1 to work around https://github.com/xunit/xunit/issues/1601
* Move xUnit to 2.4.0-beta.1.build3958

The .NET Core documentations states that hte SetMinThreads Method is available in .NET Core 1.0+. Updated instructions to identify that. Also updated the link to the new documentation.

Looks fine and valid to me, thanks! Makes perfect sense.

Note: tests folder is still different until we merge PRs and change directory structure here to /src, /tests, etc.

* Removed invalid call to SetRequestSent for IMultiMessage

* Improved (corrected) check logic.

* Disable Stop-Words added

* DisableStopWords enum value updated

* Tag feature added

Except for redisearch.io and www.ideawu.com, because they don't have an https endpoint atm.

* Added a note on .NET Core and thread counts

It threw us a bit when we saw this exception but not the same information :)

* Added details for .netstandard 2.0

* Update Configuration.md

* Alphabetical order.

It was unreachable because EndOfStreamException is inherited from IOException.

GeoTests.cs converted to UTF-8, .idea dir added to .gitignore.

Also adds Sentinel to appveyor, we hope. Or this will fail miserably.

These get overwritten when run, so they're ignored via:
git update-index --assume-unchanged RedisConfigs\Sentinel\sentinel-26379.conf

To make changes that should propagate to others:
git update-index --no-assume-unchanged RedisConfigs\Sentinel\sentinel-26379.conf

If we're AddressFamily.Unspecified, that'll always error. Assume IPv4 (as we did before).

This fixes connecting to IPv6 servers.

This adds IPv4 parsing and connect testing, verified that the fix in #686 will work.

Before converting most of the project to C# 7, `netstandard2.0`, etc. I want to get unit tests in shape. There's a fair bit of complication to doing so in that we need to test against:
- Different versions
- Clusters
- Master/Slave setups
- Multiple-test unfriendly tests (like SHUTDOWN commands)
- Simulate remote (read: latent) connections

This work does a few things:
- Moves all unit tests and assertions to xUnit
- Enables parallelization for tests
- Utilizes the xUnit output semantics, which means things like connection logs are on the test in VS, console output, build servers, etc. (it was a giant pool of `Console.WriteLine` mixed before)
- Cleans up the `\packages` folder completely (it wasn't used for anything but the `redis-*.exe` binaries
- Cleans up all test code formatting in general
- Re-enables `net462` in testing (was only `netcoreapp1.0` before)
- Allows overrides via `TestConfig.json`
- Updates all the RedisConfig\ scripts for easy test running
- Adds a StackExchange.Redis.Tests\README.md
- Fixes many tests

Remaining Long-term TODOs
- Look at Redis 4.0 on Windows via WSL
- Configure endpoints in a `.json` config override that's in `.gitignore`
- Find a way to handle master/slave breaks on underpowered machines better