rtpsrc: Does not work with playbin/playbin3 when SSRC changes

One of the use case I'm testing is the ability to leave an playbin3 instance running and playback whatever comes on using rtp:// URI. Unlike ristsrc, rtpsrc support multiple source pads, so when a new SSRC arrives, it will expose a new pad. Though the element does not have any of the require machinary (stream collection update or no-more-pads signalling) that would allow playbin to do the transition.

In ristsrc, we made the deliberate decision of having only 1 srcpad for this reason (though rtpjitterbuffer don't seems like being cleanup though). I wonder if it isn't a mistake to offer multiple pads if we can't make that work with playbin due to lack of signalling ?

To reproduce:

# Server
 gst-launch-1.0 videotestsrc ! x264enc tune=zerolatency key-int-max=30 ! h264parse ! mpegtsmux alignment=7 ! rtpmp2tpay ! rtpsink address=127.0.0.1 port=6666

# Client
gst-play-1.0 --use-playbin3 rtp://127.0.0.1:6666

# Restart the server

cc @den_erpel