Modify Ampache's trackForUrl to be ASYNCHRONOUS! YAY!

It now returns a blank MetaProxy::Track then starts a worker thread.
When the worker thread completes it updates the metaproxy track.

Big It works fine for about 10ish tracks, but for >10 tracks, the 10+ don't get updated.
Still this is a step in the right direction.

(This awesomeness brought to you buy the lovely beach of Jamaica, and by Red Stripe, authentic Jamaican Beer.)