Add CBT support, require MSBuild 15, support .NET Standard 1.4

[doubleyewdee]

Fixes:

• AssemblyVersion not in sync w/ NuGet package version.
• Adds use of CommonBuildToolset (https://github.io/commonbuildtoolset) for build extensions
• Migrate to MSBuild 15
• Target .NET Standard 1.4 in addition to 4.0, 4.5
• Some code cleanup required by migration

Stuff to look at (code-wise):

• Changed Dispose check in RecyclableMemoryStream to use long disposed member in order to switch to Interlocked.Read (required for netstandard14)
• Changed EventSource static writer to be named Writer (avoids overload confusion with new static Write methods in newer versions of EventSource e.g. netstandard14)
• Alternate (non-override) signature for netstandard14 Close and GetBuffer methods on RecyclableMemoryStream (they are not members of MemoryStream) -- the GetBuffer method is interesting, netstandard users will need to ensure they use RMS and not MS if they need to peel off large buffers for whatever reason.

[arpit-nagar]

As you already added support for netStandard in master.
Can you please publish a nuget package which supports net standard?

[brunoblank]

I tried the source today (compiled for netstandard 1.6) and TryGetBuffer did return false.
Is the netstandard support complete?