Summary:
I have need of these in the proxygen WebTransport implementation, and don't want the full dependency on QuicSocket.
Also refactored WriteCallback into StreamWriteCallback and ConnWriteCallback, leaving the original WriteCallback as both for now.
Reviewed By: hanidamlaj
Differential Revision: D63486821
fbshipit-source-id: 4b16ad871c4deac4e262c12835ad5c457e9240da
