Fined grained mpd update

The current MPDUpdate plugin send a full update command to mpd, which then scans and updates the full library directory tree. This can take a relatively long time fir big libraries, even for changes that usually impact a limited number of files.

It would be more efficient to send an update request targeting specifically files or directories that have changed or have been moved. The mpd protocol support that (see http://www.musicpd.org/doc/protocol/database.html). Unfortunately that also means that several update commands will sometimes be needed for a single command on beets side.