SocketAwaitable: Add a field for Transferred.

Changes to SocketAwaitable.Buffer affects SocketAwaitable.Transferred
and makes it unreliable after updating the buffer.

Add a new ArraySegment<byte> field `transferred` in SocketAwaitable for
holding the transferred data with no dependencies on the buffer.

Initialize the transferred field in SocketAwaiter.Complete and clear it
in SocketAwaiter.Reset.

Update SocketAwaitable.Clear so it will also clear the reference to the
transferred.Array before pooling.

Remove the warnings about using Transferred after Buffer from Readme.
SocketAwaitable: Mark UserToken as obsolete.

Add [ObsoleteAttribute] to the SocketAwaitable.UserToken property.
Remove unit tests regarding the user token.
Increase assembly version to 1.1.1.
Merge branch 'release-1.1.1' into next