Draft: rtpmanagerbad: use RTCP sink socket iso RTCP src

rtpsink was re-using the udpsrc socket to send and receive RTCP packets. When an application is changing the pipeline state from NULL -> PLAYING -> NULL -> PLAYING... udpsrc will close the socket that is shared with udpsink. As a result, only the first time the pipeline goes to playing; will it send RTCP packets, all other times, it will complain about the closed socket (but send RTP).

By re-using the udpsink socket instead of the udpsrc socket and setting the the shared socket to NULL before the state transition is applied on udpsrc, the socket can be re-used in future transitions.