@Override
  public void addExtendedAttribute(String attributeKey, byte[] attributeValue) {
    if (extendedAttributes == null) {
      extendedAttributes = new HashMap<String, byte[]>();
    }
    extendedAttributes.put(attributeKey, attributeValue);
  }
@return the cluster id on which the change has originated. It there is no such cluster, it
        returns DEFAULT_CLUSTER_ID (cases where replication is not enabled)
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/WALKeyImpl.java