* Use a participant worker queue in room.
Removes selectively needing to call things in goroutine from
participant.
Also, a bit of drive-by clean up.
* spelling
* prevent race
* don't need to remove in goroutine as it is already running in the worker
* worker will get cleaned up in state change callback
* create participant worker only if not created already
* ref count participant worker
* maintain participant list
* clean up oldState
