rtpstats: Fix packet rate calculation when multiple packets have the same RTP timestamp

The current code for the packet rate calculation inside rtpstats used by both the rtpsession and the rtpjitterbuffer currently ignores all packets where the timestamp doesn't change. This means that for video streams where all packets belonging to the same frame, it actually calculates the frame rate.. which can be quite different which high bitrate streams. This makes the computation of the max-dropout and max-misorder be completely off.

I've also exposed the packet rate in the statistics of the rtpsource to make it easier to write a test. And also easier to monitor in production.

CC @hgr