IpodCollection: finally safe solution for crash on early eject

We now (asynchronously) wait for the job that parses iPod tracks and
playlists before destroying IpodCollection.

Other change is that we factor playlists parsing into
IpodParseTracksJob, it really belongs there. (previously it was just
called from that thread and it made false impression that the method
is only called from the main thread)


FIXED-IN: 2.6