code: public static Message buildFlushRequest(ByteBuffer body) {
      ZabMessage.FlushRequest flushReq =
        ZabMessage.FlushRequest.newBuilder().setBody(ByteString.copyFrom(body))
                                            .build();
      return Message.newBuilder().setType(MessageType.FLUSH_REQ)
                                 .setFlushRequest(flushReq)
                                 .build();
    }
docs: Creates a FLUSH_REQ message.
  
  @param body the data of the request.
  @return a protobuf message.