* support setting transport to UDP
* breaking change: use new PacketContext rather than RtspMessageContext
in places where an RTP/RTCP packet is expected, as it can now be over
UDP instead of via RTSP interleaved data
* send teardown, which is important with UDP sessions. (It also will
help somewhat with Reolink TCP.) Along the way, make Session Unpin
so that teardown can consume it without tripping over tokio::pin!().
* refactor to shrink the amount of data used by Session and how much
gets memmoved around on the stack, instead of further growing it.
Because of missing RTCP RR and reorder buffer support, this is only
appropriate for using on a LAN. That's enough for me right now.
