SMW\CacheHandler

Class that handles access to cacheable entities and shielding against
a tight coupling of ObjectCache/BagOStuff in SMW

Coverage: 100%
CRAP: 21

## Note
[1] https://gerrit.wikimedia.org/r/#/c/61171

Change-Id: Ie1e4c4d0560aaf2a71adab9a3531455bcea24448