libtracker-data: Perform VACUUM only on large database files

The agressive VACUUM on shutdown is the cause for the reported slowness
in tracker-store restarts. Let's be a bit more conservative, and only
trigger VACUUMing when the database file gets hideously large (4GB).

There is the remote possibility that a database is still larger than
4GB after VACUUM. I'll just make it suck at the moment and do the same
frequent VACUUMs we get currently.