rtpjitterbuffer: Replace timer data structure
This is an effort to reduce the number of thread wakeup due to timer rescheduling and to reduce the overhead of timers lookup. In this merge request, a new RtpTimerQueue object is introduced. It's internal storage is inspired of the existing TimerQueue and replaces both the RTX timers queue and the timers GArray.
Depends on gstreamer!181 (merged)