Support synchronous handshake writes in ServerHandshakeTest.
Support synchronous handshake writes in ServerHandshakeTest.

Summary:
Previously we would only write the data to the client after an asynchronous
event.

Reviewed By: mjoras

Differential Revision: D22020222

fbshipit-source-id: cecd8196d692a0b9f0a60ab7849707bcbb0e1a20

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst