Add a simple QuicTransport server for testing and demo purposes. The server currently has two modes, echo and discard. I've changed the integration test to use the simple server instead of a session with a mock visitor. This CL also adds some of the missing APIs and fixes some of the bugs I've found while doing this. gfe-relnote: n/a (code not used) PiperOrigin-RevId: 278456752 Change-Id: Idf9aa654aa0d66673f300f2f5425f0716d6c3e14