Use estimateRowCount() instead of actually counting everything
On large wikis with lots of lint errors, counting the entire table can
be problematic from a performance perspective, sometimes taking minutes.
Instead, use Database::estimateRowCount(), which uses EXPLAIN SELECT
COUNT(*) to get an approximate value for the number of rows. We do make
sure that if the category actually has no rows, that it will return 0.
This should be considered a temporary solution, and we should look into
doing something like the SiteStats incremental updates table in the long