Make rtpsession emit collision events when a new sending source causes a collision compared to an existing remote source
This also includes a couple more commits which tests the various SSRC collision scenarios to ensure we don't have future regressions as the code is not the most trivial inside rtpsession. Scenarios added include:
- Third party collision (both with "favor-new" == TRUE and FALSE)
- Sending a packet, then getting a remote packet with the same SSRC
- Getting a remote with an SSRC then trying to send one locally with the same
- Loopback, ignore collision if our own packets are sent back to us.