Additional benefits are:
 - ssrc -> source name will be updated on ssrc remap message from
 the bridge.
 - the map will be correctly cleaned when member leave (this logic was
 not working well for _sourceName map)
 - Looks cleaner and simpler.

* fix(SignalingLayer) Update SSRC owners on leave.
Update the SSRC owners in the following cases:
1. When a remote endpoint leaves the call.
2. When a source-remove is received.
3. When a source is remapped (with ssrc-rewriting enabled).
Create the remote track even if presence is not yet received. The ssrc owner check prevents the client from creating a dummy track when the call switches over from p2p to jvb when the last remote endpoint leaves the call.

* ref(SignalingLayer) alpha sort methods.
Clean up unused methods, _findEndpointSourceInfoForMediaType is not used anymore.

* squash: Address review comments.

It does the minimum necessary to function without the complexity of the
main one.