Move from db/DatabaseGlue.cxx, eliminating global variable.

Zero means "unknown".

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Replaces GLib's GError.

Use the C++ API.

Optimize the ProxyDatabase by invoking "list" on the peer, instead of
visiting all songs.

Implements error checking for the gmtime_r() call, which was missing
in two code locations.

Let the database plugin do the match.

Merge locate_song_search() and locate_song_match().

Fold the case during construction, without having to create another
copy.