Set version to '2.1'

Merge remote-tracking branch 'origin/v2.0'

Merge pull request dotnet#64 from AArnott/fixMultiplexingHandshake

Avoid write-and-flush without awaiting

Start building 2.1-beta

Merge pull request dotnet#57 from AArnott/prefixingWriter

Fixes and more tests for PrefixingBufferWriter

Ship stable versions out of v2.0

Adjust how Sequence<T>.MinimumSpanLength works

The default minimum is now to allow whatever the pool recommendation is.

Merge pull request dotnet#55 from AArnott/fix52

Add Sequence<T>.MinimumSpanLength

Merge pull request dotnet#54 from AArnott/prefixingWriter

Add PrefixingBufferWriter

Merge pull request dotnet#50 from AArnott/AArnott-patch-1

Update System.IO.Pipelines to 4.5.3