Add caching to looking up totals

The query itself is too expensive to be run on large Wikimedia wikis. So
put it behind memcache and increment/decrement the count for each
category as we add and delete errors from the database.

If this happens to get out of sync, it will get fully refreshed
regularly when the totals are sent to statsd.

Change-Id: I3774103a29fa0f29d36283950f136259fa71bffe