RTP collision design


GstRTPCollision
---------------

Custon upstream event which contains the ssrc marked as collided.

This event is generated on both pipeline sender and receiver side by
the gstrtpsession element when it detects a conflict between ssrc.
(same session id and same ssrc)

It's an upstream event so that means this event is for now only
useful on pipeline sender side. Because elements generating packets with the
collided SSRC are placed upstream from the gstrtpsession.

rtppayloader
------------

When handling a GstRTPCollision event, the rtppayloader has to choose another
ssrc. 


BYE only the corresponding source, not the whole session.
---------------------------------------------------------

When a collision happens for the given ssrc, the associated source is marked
bye. But we make sure that the whole session is not itself set bye.
Because internally, gstrtpsession can manages several sources and all have
their own distinct ssrc.