EntityRevisionLookup is not an EntityLookup

EntityRevisionLookup should not extend EntityLookup. Instead,
implement EntityLookup on top of EntityRevisionLookup.

This means that the different implementation (and proxies) of
EntityRevisionLookup don't need to implement EntityLookup,
reducing overhead.

Also, the contracts of the two interfaces are planned to diverge
in the future, with EntityLookup silently resolving redirects,
and EntityRevisionLookup handling redirects explicitly.

Change-Id: I5b3b3945a0f48d9fc4a208acc0087a23627b556f
Merge "EntityRevisionLookup is not an EntityLookup"