Instead of the lower level TrackerDataManager object directly.
The only additional thing that tracker-store does is signal
emission for writeback and GraphUpdated, the internal
TrackerDataManager object is still accessed to implement those
features. This makes libtracker-direct the only place where
queries/updates are queued, performed and dispatched.

There's other indirect benefit from this, update queue handling
no longer needs to hit the main thread in order to schedule the
next update. Besides the very unlikely thread contention situations
described in previous commits, this should maximize throughput
of the updates queue.