WIP: Wire up media pipelines

Here is a first cut at wiring up the pipelines. This will do a video call and pass some of the unit tests.
This is probably OK for now, but we need to think about using UUID keys for these with a lookup table.  We can't rely on the msid we receive from the peer being unique, so we'll need our own identifiers.

Changing pc_track_id to a string (or nsID) might be the best way to start here, since you can see what that touches with the help of the compiler.