Make ReferencedEntityFinder use Entity::getAllSnaks

Note how this reduces the complexity of ReferencedEntityFinder
in preparation of the creation of more such classes, e.g.
ReferencedUrlFinder, ReferencedImageFinder, etc.

Change-Id: Ie9fcbb315f858e36566bc6f702c659d241d5679d
Merge "Make ReferencedEntityFinder use Entity::getAllSnaks"