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.

The reader and the writer now communicate through a channel and the packet
cache.  If the writer thread drops behind, we drop packets after inserting in
the packet cache, which avoids building a backlog.

For now, this is only used to request screen sharing as opposed to normal
videos.  In the future, it will be used for simulcasting.

Commit 8257a7ff "Remove dead variables." was wrong in that it deleted
the 'diconnect' [sic] variable definition, and left the
'disconnectbutton' variable undefined.

I'm an idiot.

We used to deadlock with large numbers of tracks.  This should fix that.

Thanks to Antonin Décimo.