Cache items found in ClientSiteLinkLookup

Caches the items which have been found by title in the ClientSiteLinkLookup
in an array and makes it available through the WikibaseClient default
instance. This improves performance a lot.

Change-Id: I0b7477aead578384f3d1331fc984e2ce48bb9e28
Merge "Cache items found in ClientSiteLinkLookup"