CachingEntityRevisionLookup has existed in Wikibase for a long time.
Its main function is to interface with a shared cache for entity revisions, which is shared between ALL sites of a cluster.
With some of the code written for MCR and RevisionStore refactorings, MediaWiki core now has a BlobStore cache that is shared across the whole cluster.
Since then Wikibase is storing all retrieved entities twice in the cache.
We can now probably remove the layer of caching in Wikibase, but should first evaluate exactly what that will mean in terms of changed db calls etc.
Perhaps `CachingEntityRevisionLookup` will remain, as the entity revisions are still cached, but the cache is somewhere else.
- Reduce Memcached usage in production
- Reduce cache usage for 3rd party wikibase users
- Reduce code complexity in Wikibase