rtpbin: Make cleaning up sources in rtp_session_on_timeout MT safe

rtpbin: Make cleaning up sources in rtp_session_on_timeout MT safe

Using _foreach_remove on the hashtable, while releasing the lock protecting
that table inside the callback is not a good idea. The hashtable might
then change (a source removed or added) while signals like on_timeout
are being sent.

This solution makes a copy of the table, performs the _foreach without
actually removing any sources, but marks them for removal on a second
iteration with the real list, but this time not letting go of the lock.

Fixes #630452