Improvements to how we track activity on the backend.

The current activity tracking system is based heavily on the similar subreddit activity counts.  However, since we now have a cron job that counts activity and broadcasts it on sockets, we can take advantage of that to remove the memoize and fuzz-caching stuff and drastically simplify the in-request behaviour. The cron job will, in addition to its previous duties as socket broadcaster, update an attribute of the event itself which is used in-request.  If Cassandra slows down on the `get_count`s for some reason, it will no longer have any effect on live requests.

This also made it super easy to add a time-series CF for historical activity numbers. 

:eyeglasses: @alienth