- add more tracking levels to read-state
- if PreventThreadTheft feature flag is enabled, explicitly push continuations to the thread-pool

Note that we're only building `StackExchange.Redis` and `NRediSearch` in the CI build so this wasn't apparent as an issue, but it's also easily remedied. Cleaning up the xUnit MyGet feed as well since it was an intermittent issue earlier in testing this...and turns out we don't use any betas or need them anymore, so why not simplify anyhow.

Overall, this is a simpler alternative to #1313 and more build simplification/cleanup:

Total changes:
- Moves to 3.x build SDK (just to update)
  - Moves to `Visual Studio 2019` AppVeyor build image, for 3.x support (removes need to install SDK)
  - Rolls forward to latest version
- Moves to `Build.csproj` for building
  - Simplifies `build.ps1`
- Adds `NRediSearch` to the test suite (on Linux in CI, since Windows is against 3.x)
  - Adds `RedisFeatures.Module` (against v 4.0.0) to guard tests against
  - Makes `NRediSearch.Tests` reference `StackExchange.Redis.Tests` and support test skipping
- Makes copyright year range dynamic for the library
- Moves `LangVersion` to latest and in one place (root `Directory.Build.props`)
- Uses `Microsoft.NETFramework.ReferenceAssemblies` to build `net4x` on Linux
- Uses `IsPackable` to default to excluded then reverses to `true` only for the `src/` directory
- Removes `xUnit` MyGet feed, since we're on stable builds now
- Adds new files (and corrects old) in `.sln`
- Removes `LibTargetFrameworks` to simplify since it's less complex given the above changes

cc @mgravell for eyes - this should be all set now!

These got left behind in root due to being ignored in git, needed a good 'ol `git rm -r --cached RedisConfigs/Sentinel/` here.

Fixes #93.

The previous link was relative to the document, however, when displayed in GitHub Pages it ends in a 404. I've changed to point to the source file in GitHub UI.

minor text fixes.

Extending description of the IServer.Time and IServer.TimeAsync by specifying the timezone of the return values (#1311)

Updated documentation to reflect that PreserveAsyncOrder is deprecated and ChannelMessageQueue should be used instead

Adds LATENCY and MEMORY command support and testing.

Implements #1238 - adds `HSTRLEN` command and testing.

This reverts commit a683d9c8.

dedicate_ thread-pool is almost fully available --> dedicated thread-pool is almost fully available

* Accurate timespan if the stopwatch is high resolution

* Made helper class internal instead of public

Build images have slipped again, testing a rev to latest stable SDK generation and manually installing on Windows since Appveyor doesn't have it.

Including v2 API mentioned in #639 for both synchronous and asynchronous handlers.

* Added documentation for the Stream data type.
* Updated to account for the `MKSTREAM` option now used in the `StreamCreateConsumerGroup` methods.

minor logging tweak re #1114 - make sure that *command* is logged even when a base-error-message is provided; either way, *only* report the Command, not the CommandAndKey (we will add that later if detail is permitted)

* add failing test for #1103

* add explicit ulong handling into RedisValue

* Fix (i.e. document and fix incorrect test assertions) the change re "-"/0; add ulong support to RedisResult

* add extra tests for +/.

fix #1108 - introduce LogProxy as an intermediary between the TextWriter; move the sync to there - allows safe detach from the logging (#1116)

* rework how pubsub handlers/queues are stores and activated
- switch to linked-list for queues
- use alloc-free path for all handlers invokes

* defer writing to the queues until we've released the subscriber lock

additional for #1101 - make sure the queue gets marked as completed when unsubscribing; simplify the way unsubscribe works for CMQ - remove the ForSyncShutdown concept and just mark completed directly

test and fix for #1101 - Unsubscribe[Async] with null arg should wipe both trusted and non-trusted subscribers