Cache countable statistics to prevent multiple counting on import

At the moment, when $wgArticleCountMethod = 'link' (as it is on the WMF
cluster), we are querying the Slave database before each individual
revision is imported, in order to find out whether the page is countable
at that time. This is not sensible, as (1) the slave lags behind the
master, but (2) even the master may not be up to date, since page link
updates take place through the job queue.

This change sets up a cache to hold countable values for pages where import
activity has already occurred. That way, we aren't hitting the DB on every
revision, only to get an incorrect response back.

Bug: T42009
Change-Id: I99189c82672d7790cda5036b6aa9883ce6e566b0