This will hopefully become unnecessary once we have better congestion
control.

JSDoc doesn't recognise them otherwise.

Thanks to Rémi Nollet and Léo Stefanesco.

For now, join must follow login, but it will make it easier to extend
the protocol for joining multiple groups (think federation).

No need to carry password around.

More reliable reaction to network failures when they are detected
by just one side.

This makes it easier to deploy by putting all user-serviceable files
under data/.

REMB applies to the whole transport, not to individual tracks.