code: public static Message buildDelivered(Zxid deliveredZxid) {
      ZabMessage.Zxid zxid = toProtoZxid(deliveredZxid);
      ZabMessage.Delivered delivered =
        ZabMessage.Delivered.newBuilder().setZxid(zxid).build();
      return Message.newBuilder().setType(MessageType.DELIVERED)
                                 .setDelivered(delivered).build();
    }
docs: Creates a DELIVERED message.
  
  @param deliveredZxid the last zxid of delivered transaction.
  @return a protobuf message.