rtpjitterbuffer: With do-retransmission and rtx-next-seqnum, wakes up the timeout thread for every packet that arrives

Submitted by Sebastian Dröge @slomo

Link to original bug (#751846)

Description

update_timers() will remove the current timeout for the current packet and add a new timeout for the next packet. That will unschedule the current timeout, timeout thread wakes up, waits for the next timeout. Goto beginning.

This means we're doing a lot of context switches in the good case when no packets are lost and no reordering happens.