Also don't rely on tracks being immutable in sendRR.

Don't send SRs for tracks for which we have no time offset yet.
Send an unscheduled SR when we get our first time offset.

This was broken when we pushed user notifications into addClient.

Since the packet entries are already overdimensioned, there's no
need to add additional padding.

In principle, we should be receiving receiver reports only, but let's
be robust.

This avoids locking in sendRR.

This fixes leaking of rtpUpSender goroutines.

This may happen if we close a connection before getting any tracks.