Made getMaxLag() use caching to reduce connection spam

* This method is currently hit when bots use the maxlag API parameter
* Also added a $fallback param to ObjectCache::newAccelerator so code
  does not have to keep reinventing that logic.

Change-Id: I45bf1085301c5cf69b29b494a146c6a067534581
Merge "Made getMaxLag() use caching to reduce connection spam"