gst/rtpmanager/gstrtpsession.c: Implement forward and reverse reconsideration.

gst/rtpmanager/gstrtpsession.c: Implement forward and reverse reconsideration.

Original commit message from CVS:
* gst/rtpmanager/gstrtpsession.c: (rtcp_thread):
Implement forward and reverse reconsideration.
* gst/rtpmanager/rtpsession.c: (rtp_session_get_num_sources),
(rtp_session_get_num_active_sources), (rtp_session_process_sr),
(session_report_blocks):
* gst/rtpmanager/rtpsession.h:
Small cleanups.