This colossal commit enables async trackForUrl support for mp3tunes, however to achieve this MetaProxy was hacked to allow for selective updating of the proxied track.

Due to the inheritance hierarchy in the design a chicken and egg problem developed where MetaProxy::name() would call itself causing infinite recursion.

The solution was to add a "forwardToProxy" flag to MetaBase that by default does nothing, but is implemented in MetaProxy and ServiceMetaBase to allow for selective getting of cached or real metadata.