* Perform unsubscribe in parallel to avoid blocking
When unsubscribing from tracks, we flush a blank frame in order to prepare
the transceivers for re-use. This process is blocking for ~200ms. If
the unsubscribes are performed serially, it would prevent other subscribe
operation from continuing.
This PR parallelizes that operation, and ensures subsequent subscribe
operations could reuse the existing transceivers.
* also perform in parallel when uptrack close
* fix a few log fields
When the publisher stops publishing, the individual receivers would close
attached DownTracks first before notifying MediaTrackReceiver callbacks.
This means #1454 does not fix the issue entirely since there is still a window
when we can subscribe to a closing track.
cnderrauber