Includes
- Implementation in src/outputs/airplays2, type OUTPUT_TYPE_AIRPLAY
- Homekit pairing, both normal (with PIN) and transient
- New session startup sequence, incl GET /info, SETPEERS and 2 x SETUP
- No more OPTIONS and ANNOUNCE
- Use POST /feedback for keepalive instead of SET_PARAMETERS
- Sequence dispatching instead of callback chains
- Continue despite "Bad request" to SET_PARAMETER (volume)
- Opening of event connection to receiver (reverse rtsp connection)
Still to be done
- Password authentication
- Handling of events
Old Airport Express hangs up if we use GET_PARAMETER (despite announcing support
for it...), and since iTunes seems to use OPTIONS we also do that. We weren't
really using the return value of GET_PARAMETER anyway.
See also comment added to commit d9a67b6dcedef61a37d67e996262bcad4afed157
