rtspsrc/rtpbin: Doesn't sync A/V if the server does not provide RTCP

The A/V synchronization only happens in rtpbin when it receives an RTCP SR from rtpjitterbuffer. In case of RTSP this could already happen once the caps are received with clock-base / npt-start, and the first packet is received to know the local arrival times. The clock-base is the RTP timestamp of each stream that corresponds to npt-start.

If the server never sends an RTCP SR then A/V sync is never fixed up, and even if the server sends an RTCP SR it would be much later than necessary.

This can probably be solved by adding a new variant of the sync signal to rtpjitterbuffer that provides clock-base and npt-start among other things and has rtpbin handle this as a special-case in gst_rtp_bin_associate().