* Remove callbacks queue from sfu/DownTrack
- Connection stats callback was happening in connection stats go routine
- RTT update launches a goroutine on the receive side as it affects the
subscriber. So, no need to queue it.
- Changed two things
o Move close handler to goroutine. It is better that way as it touches
the subscriber as well
o Move max layer handling into a goroutine also so that the callback
does minimal work.
With this all the send side callback queues are removed.
* small clean up
2f902a3edc