Making trackForUrl asynchronous?

[Dan Meltzer]

Hi Friends,

After seeing the way ampache currently handles trackForUrl it seems
like it may make sense to make it asynchronous (or add a similar
asynchronous method such as fetchTrackForUrl.)  This doesn't seem like
it would be too difficult, we could either return MetaProxy's until
the track was fetched or simply not return a track at all and emit a
signal when its fetched.

Thoughts?