This fixes leaking of rtpUpSender goroutines.

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

It would be better to terminate FIR, note that a keyframe was requested,
and send FIR on our own.

Thanks to Giuseppe Castagna for noticing.

It was racy, and it drops the chat history.  Let them accumulate for now.

This spuriously breaks client-to-server connections over IPv6.
Thanks to Fabien de Montgolfier for his help with debugging this.

It turns out that track ids are not necessarily the same on the local and
remote sides.  Thanks to Ines Klimann for noticing the issue.