mpd: Install new commands helpers in _call_handler

This means we now tokenize every request, and then try in call the appropriate
handler. If none is found we simply fall back to the old handlers.
mpd: Store auth required and if command should be listed
mpd: Cleanup imports in reflection
mpd: Upate command reflection to handle new commands helper
mpd: Convert protocol.status to new helper
mpd: Convert protocol.playback
mpd: Add protocol.position_or_range converter
mpd: Convert almost all of protocol.current_playlist