http.FileServer is inflexible, it is better to reimplement it
rather than trying to work around its limitations.

Keeps go vet from complaining.

Move RTP-specific code into its own file.

This requires a proper parser for commands.

Users find it confusing, especially when the browser disables autoplay.

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.