NoteDb: Consider starred changes part of the Accounts table

A StarredChange is a simple pair of (Account.Id, Change.Id), and will
ultimately be stored in NoteDb in the All-Users table. It turns out it
is not even stored under the Change entity group in the
googlesource.com backend, meaning among other things that starred
changes can't be updated in the same transaction as a Change entity.

Add a new value ACCOUNTS to the NoteDbTable enum and teach
NotesMigration to understand this table. Note that in practice we've
started migrating Accounts to All-Users without the use of
NotesMigration, which is why we haven't done this yet. It's even
possible that we will migrate StarredChanges in the same way, but
that's a decision for later. This change is the minimal work necessary
to stop considering StarredChanges part of the changes migration.

Use the new migration bits within StarredChangesUtil. Remove support
for starred changes from ChangeRebuilder (no longer applicable) and
RebuildNoteDb (not implemented yet).

Change-Id: I84fa8b78d308b7dcd6b7592cea8898a93135cd2c

gerrit-server/src/main/java/com/google/gerrit/server/notedb/NoteDbTable.java

@@ -15,6 +15,7 @@
 package com.google.gerrit.server.notedb;
 
 enum NoteDbTable {
+  ACCOUNTS,
   CHANGES;
 
   String key() {