websocket: Make WSH.get a coroutine

websocket: Make WSH.get a coroutine

This is necessary to convert accept_connection to native coroutines -
the handshake no longer completes within a single IOLoop iteration
with this change due to coroutine scheduling.

This has the side effect of keeping the HTTP1Connection open for the
lifetime of the websocket connection. That's not great for memory, but
might help streamline close handling. Either way, it'll be refactored
in a future change.