Use NoteDb for ref filtering when asking only for a single change ref

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

java/com/google/gerrit/server/git/SearchingChangeCacheImpl.java

@@ -45,6 +45,13 @@ import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.ExecutionException;
 
+/**
+ * Cache based on an index query of the most recent changes. The number of cached items depends on
+ * the index implementation and configuration.
+ *
+ * <p>This cache is intended to be used when filtering references. By design it returns only a
+ * fraction of all changes. These are the changes that were modified last.
+ */
 @Singleton
 public class SearchingChangeCacheImpl implements GitReferenceUpdatedListener {
   private static final FluentLogger logger = FluentLogger.forEnclosingClass();