Issue: Add Area iCache to the Datastore

We've taken a hint from criticisms of vBulletin storing the forumcache in the datastore, so we're not saving the area cache to the datastore. This saves memory on wiki pages by not having all of every area's settings loaded all the time.

However, when we do need areas, such as when we need permissions - whether inherited for a specific page or in totality for special pages - and this does happen on every page, we then need to make 2 database queries... Once for the parentlist or area IDs, then again for the area info or permissions.

We could eliminate 1 database query here if we created a datastore entry for the icache - that is, the list of area ID numbers, organized by hierarchy and in the order they appear in lists. Compared to the full data, the IDs in a list would likely be only a couple hundred bytes (< 1 KB).

Too bad we can't do this for more common content types (like Books and Categories)...