Direct streaming via apollo

[tsani]

This turns apollo itself into the server for the HTTP music stream, instead of leveraging MPD. This means we have more flexibility in the way we integrate the HTTP API with the music stream.

[tsani]

As a stop-gap until we fully replace MPD, we can delegate the work of proxying the MPD HTTP stream to Apollo instead of having nginx do that separately.

Note that if we do this, we should make it so that if the backing MPD instance is not in the playing state when a client connects to apollo, that it enters the playing state.