disorder-rescan now obsoletes tracks that belong to no collection;

disorder-rescan now obsoletes tracks that belong to no collection;
handy when you remove some collection.  The code should be relatively
robust against such tracks but they'll annoy the random track picker.

trackdb_rescan() is thus modified to be able to return all tracks.

find_track_collection() maps a track to its collection, or 0 if it
doesn't have one.