rtpmanager: Add support for in-band synchronization (RFC 6051).

Submitted by GstBlub

Link to original bug (#784132)

Description

Created attachment 354323
rtpmanager

This allows endpoints to synchronize instantly rather than having to wait for the first RTCP SR.

Patch 354323, "rtpmanager":
0001-rtpmanager-Add-support-for-in-band-synchronization-R.patch