DAR-1061 Added caching layer to `WallHistory::getLastPosts()`.
DAR-1061 Optimized query in `WallHistory::getLastPosts()`.
Merge pull request #1111 from Wikia/DAR-1061

DAR-1061 Performance problems with Forum Activity Module

We're going to merge this due to the drastic performance improvement and the user facing nature of it (users were complaining).  We are sending an email to the QA team to request additional regression testing on the Forum features.