pootle_store: Add database cache columns for wordcounts and suggestion count
pootle_store/caching: Update total_wordcount on Unit.save()
pootle_store/caching: Update wordcounts when creating a new Unit
pootle_store/caching: Update Store caches when saving an existing unit
pootle_store: Get rid of calc_*_wordcount
commands: Update refresh_stats to actually refresh stats