Adding support for netstandard2.0

[lstratman]

First off, excellent library, thanks so much for making this available. You are miles ahead of any other C# VNC library out there and the integration work necessary to drop this into EasyConnect was straightforward and well designed.

This pull request does a few things:

1. Adds support for netstandard2.0, which allowed me to use this library in a .NET Framework application. Some of the Span-based API shims necessary for this aren't quite as efficient as their .NET Core counterparts, but they still use ArrayPools to reduce allocations and perform perfectly well in my testing.
2. Adds support for the ClientCutText message type. Figured I would take care of this while I was in there messing about.
3. Calls to CloseAsync() no longer throw uncaught exceptions when winding down the message threads. I made a few changes not to throw exceptions when the message thread workers detect cancellation and to to swallow OperationCancelledException, but if there is a better way to close a connection (while not exiting the entire application), feel free to let me know.

Once again, thanks for all of your hard work on this library!