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
pootle_store: Only calculate difference once
pootle_store: Do not calculate target_wordcount, ever

count_words is an expensive operation, and we don't really use
target_wordcount anywhere important.
Where it is actually used can be dropped in later commits.