SearchingChangeCacheImpl only returns a fraction of all changes. This
fraction covers most recently modified changes. This is problematic when
users want to fetch a change that is older than what this set covers.
There are Gerrit instances with millions of changes and the concept of
index-backed evaluation doesn't scale to this size. Other concepts
failed and had to be reverted (I447eaeb).
If refs-in-wants on Protocol V2 is used, we can fix the shortcomming by
investigating only a single ref backed by NoteDb.
Change-Id: Ib01492ac58eda4a37086aa3e106be857cfb566e1
e52b9b5f74