Implement latest MPD protocol v0.22

[BrainStone]

The current protocol version of MPD is 0.22. And as many clients have implemented it already, they hardly can use mopidy at all. Anything that's beyond basic commands like play/pause, skipping forwards and backwards, volume control and replayer/shuffle/consume status is not working due to the big differences in the protocols.
Important stuff like retrieving the track list, albums, searching, etc.

It would be very nice if mopidy could get its long overdue protocol update (I mean even 0.19 is only partially implemented since at least October 2015 (almost 4 years without a protocol update), accoding to #2)

[kingosticks]

Last time I checked it was our complete lack of support for group_by that was causing the most friction. That would be the place to start, particularly before Mopidy v3.0 if support requires API changes.