tracker-data-manager.c Cleanup - part 2

This contains all the commits in !414 (merged) and !415 (merged) but splitting up the merge requests may help review. I'll rebase this one when the other will be merged. Please let me know if you think that individual patches are too small or if you prefer smaller or larger merge requests. I hope the code is becoming easier to understand. I think I found a small bug (see last patch in the series).