libtracker-data: Add consistency check

The dreaded database refcounting issue is still rampaging in some users. We can't detect all databases that are potentially broken, (eg. refcounting still "corrent", but off), but we can detect the fact that it already broke in a more elegant way than a myriad seemingly unrelated breaks.

If we have something in the rdfs:Resource table that is not in the Resource table, that's a clear indication the database is already irremediably broken. Just drop the database and reindex from scratch in that case.