libtracker-data: Clean up stale URIs on startup

The Resource table can be left populated with stale resources, in order to
avoid these from accumulating, clean these up during startup.

This assumes all miners and clients will be educated and will clean up all
references to deleted resources (triples of the form {?urn some:prop ?deleted})
at the moment deletes happen, and always during the lifetime of the previous
run of tracker-store.

If there's sloppy miners/clients that don't fully unlink the resource, or
users mess with the lifetime of tracker-store in the middle of such
maintenance operations, we will leave such stale values around the
database, possibly confusing queries where the predicate is used in the
WHERE section. Short of having fully refcounted resources, seems like a
minor drawback.