* 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.
* stop using deprecated failure crate and its deps
* more uniformly detailed error messages
* an enum of the type of error, currently internal-only. I'm not
confident enough I understand what cases a caller might want to
distinguish, so I added a comment inviting input instead of
exposing it now.
* while I'm at it, separate connection context and message contexts.
This shrinks some of the data memmoved around in PacketItem and
CodecItem.
