Enable RepoSequence with a config option

When noteDb.changes.sequence is true, read changes from the
refs/sequences/changes blob in All-Projects. Use a lazily evaluated
Seed that reads the next change number from ReviewDb and uses it as
the first number in NoteDb.

In a single-server setting this is sufficient, but in a multi-master
environment it may be desirable to leave a gap after the next
ReviewDb counter to account for the fact that not all servers may
receive the configuration update immediately. Control this with an
undocumented config option.

Change-Id: I2de8fbd42b525a75b3cdbcdb4b3f9e33e382b7a7

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

@@ -62,6 +62,15 @@ public abstract class NotesMigration {
    */
   protected abstract boolean writeChanges();
 
+  /**
+   * Read sequential change ID numbers from NoteDb.
+   * <p>
+   * If true, change IDs are read from {@code refs/sequences/changes} in
+   * All-Projects. If false, change IDs are read from ReviewDb's native
+   * sequences.
+   */
+  public abstract boolean readChangeSequence();
+
   public abstract boolean readAccounts();
 
   public abstract boolean writeAccounts();