Move RTP-specific code into its own file.

Also export some fields as thread-safe methods.

These are specific to webclients.

It's not obvious that it's useful, especially now that the writer
is split into multiple threads.  Do retain the dynamically computed
delay in case of audio congestion.

Otherwise recording to disk causes a deadlock.

This improves both scaling and fairness in the case where a single
track is being sent to large numbers of receivers.  We avoid
communication overhead by growing the number of threads as the
square root of the number of receivers.

Avoids a race.

Kick can no longer be called with groups locked.

This gives the user a friendly message when the server shuts down.

Failed media must still be usable.

We used to display the help message too early, which caused us to
display it even if the group doesn't exist or the user doesn't have
the present permission.