Refactor caching for EntityRevisionLookup service.

This makes WikiPageEntityLookup oblivious of caching,
and uses CachingEntityRevisionLookup as a decorator
(twice) to provide two layers of caching (persistent
and in-process)

Change-Id: I08f2c6d154da10d0504c86cc18d35f36d800375e
Merge "Refactor caching for EntityRevisionLookup service."