audio: Postpone tags until after stream-start
audio: Postpone tags until after stream-start

When a new URI gets set we create a pending tags dictionary. This gets all the
tags until stream-start, at which point they are all emitted at once. During
track playback tags works as before.

This ensure we don't prematurely tell clients about metadata changes.

kingosticks/mopidy,jodal/mopidy,ZenithDK/mopidy,kingosticks/mopidy,vrs01/mopidy,adamcik/mopidy,adamcik/mopidy,jcass77/mopidy,jcass77/mopidy,ZenithDK/mopidy,jcass77/mopidy,mopidy/mopidy,mopidy/mopidy,mopidy/mopidy,ZenithDK/mopidy,jodal/mopidy,ZenithDK/mopidy,adamcik/mopidy,vrs01/mopidy,vrs01/mopidy,vrs01/mopidy,jodal/mopidy,kingosticks/mopidy