code: public static Message buildJoin(Zxid lastZxid) {
      ZabMessage.Zxid zxid = toProtoZxid(lastZxid);
      ZabMessage.Join join =
        ZabMessage.Join.newBuilder().setLastZxid(zxid).build();
      return Message.newBuilder().setType(MessageType.JOIN).setJoin(join).build();
    }
docs: Creates a JOIN message.
  
  @return a protobuf message.