Myth Protocol: add a MUSIC_TAG_UPDATE_VOLATILE command

This command will be used to update the rating, last played time and play
count in a music tracks tag.

format:-
MUSIC_TAG_UPDATE_VOLATILE <hostname> <songid> <rating> <playcount> <lastplayed>
MusicPlayer: use the myth protocol to update the volatile metadata
Merge remote-tracking branch 'upstream/master'