VisibleRefFilter is using secondary index to evaluate the visibility of
refs which cannot work in slave mode since there is no secondary index.
SearchingChangeCacheImpl is the class used by VisibleRefFilter and it's
nullable so bind it to null in slave mode to force VisibleRefFilter to
not rely on that cache.
Change-Id: I5af65f0992b317b4bbd3ce050f3a7907aad713a4