Speed up -updateCrossPersistentRootReferencesToPersistentRoot: by removing loop over all persistent roots, only fix up object graphs that have references to the target persistent root.
Speed up -updateCrossPersistentRootReferencesToPersistentRoot: by removing loop over all persistent roots,
only fix up object graphs that have references to the target persistent root.

etoile/CoreObject,etoile/CoreObject,etoile/CoreObject,etoile/CoreObject