rtpjitterbuffer: Cancel timers instead of just unlocking loop thread
When the queue is full (and adding more packets would risk a seqnum roll-over), the best approach is to just start pushing out packets from the other side. Just pushing out the packets results in the timers being left hanging with old seqnums, so it's safer to just execute them immediately in this case. It does limit the timer space to the time it takes to receiver about 32k packets, but without extended sequence number, this is the best RTP can do.
This also results in the test no longer needed to have timeouts or timers as pushing packets in drives everything.
Note that this strictly depends on !252 (merged), I think in the 1.16 branch, I'll suggest just removing that unit test.
Fixes #619 (closed)