A QUIC endpoint is a UDP socket which multiplexes QUIC sessions, which
may be initiated in either direction. This PR exposes endpoints and
moves things around as needed.
Now that endpoints can be reused between client connections, we have a
way to share tls tickets between them and allow 0rtt. This interface
currently works by conditionally returning a promise.
Also cleaned up the rust op names, fixed some lingering problems in the
data transmission, and switched to explicit error types.
ccd375802a