code: public static Message buildAck(Zxid zxid) {
      ZabMessage.Zxid zzxid = toProtoZxid(zxid);
      Ack ack = Ack.newBuilder().setZxid(zzxid).build();
      return Message.newBuilder().setType(MessageType.ACK).setAck(ack).build();
    }
docs: Creates a ACK message.
  
  @param zxid the zxid of the transaction ACK.
  @return a protobuf message.