Re-use top level registries everywhere.

By re-using the top level registries (WikibaseRepo and
WikibaseClient) we allow the service objects themselves
to be re-used. This way, local (in-process/in-memory)
caches can be used effectively.

This should help to reduce the number of (redundant)
requests to memcached.

Change-Id: Ifa80245c93ce19d746f6af6efcf013ea859a3b9f
Merge "Re-use top level registries everywhere."