elements_rtpbin.test_eos occasionaly fails

(rtpbin:14679): GLib-CRITICAL **: 22:23:12.540: g_hash_table_foreach: assertion 'version == hash_table->version' failed

The original cause of this is https://bugzilla.gnome.org/show_bug.cgi?id=532011 (CC @ocrete).

What happens is the following:

1. We start generating a RTCP packet on timeout (rtp_session_on_timeout())
2. We call generate_rtcp() for each item for the SSRC hash table (this is the iteration that will break)
3. We call into session_start_rtcp() and rtp_source_get_new_sr()
4. rtp_source_get_new_sr() among other things calls get_clock_rate() which calls back into rtpsession's source_clock_rate(), which... releases the session lock in the middle of the iteration from 2)
5. In that short window, we process an RTCP packet (rtp_session_process_rtcp())
6. We call into rtp_session_process_rr(), which calls into obtain_source() and that creates a new source and adds it to the SSRC hash table (see 2))
7. A bit later, 4) continues, takes the session lock, tries to do another iteration of the hash table and 🎆

I don't know how to fix this without potentially introducing deadlocks. This is going through too many layers while assuming that the mutex is still locked (which it shortly isn't).