Start migration to notedb implementation of PatchSetApprovals

Encapsulate the current state of the migration in a NotesMigration
class. For each database table (or broader group of functionality)
that we migrate to notedb, configure a boolean indicating whether that
data should be read from notes, defaulting to false.

Unlike reads, NotesMigration contains just a single boolean
indicating that data should be written. We don't attempt to write
just some types of data; as the migration continues we will just
rewrite history.

Since most existing reads of the PatchSetApprovals table have been
migrated to ApprovalsUtil, most implementation changes happen there.
There are a few other implementations scattered around, and some that
will require a bit more work (e.g. stamping normalized approvals at
submit time).

Change-Id: I5676267d4de607c385e8c9917a89333863b9c9e7

gerrit-server/src/main/java/com/google/gerrit/server/change/RevisionResource.java

@@ -21,6 +21,7 @@ import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.PatchSet;
 import com.google.gerrit.server.IdentifiedUser;
+import com.google.gerrit.server.notedb.ChangeNotes;
 import com.google.gerrit.server.project.ChangeControl;
 import com.google.inject.TypeLiteral;
 
@@ -53,6 +54,10 @@ public class RevisionResource implements RestResource, HasETag {
     return getControl().getChange();
   }
 
+  public ChangeNotes getNotes() {
+    return getChangeResource().getNotes();
+  }
+
   public PatchSet getPatchSet() {
     return ps;
   }