This is an analogous problem to the one with search results fixed in
Ie54b9e2d. VisibleRefFilter is a little trickier because it was using
ChangeCache, which only returned Changes, not the reviewer set. Change
SearchingChangeCacheImpl to cache a different value type and return
ChangeDatas from its search method so we can use the cached
ReviewerSet where appropriate.
Unfortunately for the Reindex program we still need to support not
having a SearchingChangeCacheImpl, which means we have a completely
separate codepath for reading changes from the database.
Change-Id: Ic432a8e48a2bafc8d142b84b25101d95ffb674b7
a758640bc3