code: public static Message buildFlush(Zxid lastProposedZxid, ByteBuffer body) {
      ZabMessage.Zxid zxid = toProtoZxid(lastProposedZxid);
      ZabMessage.Flush flush = ZabMessage.Flush.newBuilder()
                                               .setZxid(zxid)
                                               .setBody(ByteString.copyFrom(body))
                                               .build();
      return Message.newBuilder().setType(MessageType.FLUSH).setFlush(flush)
                    .build();
    }
docs: Creates a FLUSH message.
  
  @param lastProposedZxid the last proposed zxid before the FLUSH.
  @param body the data of the request.
  @return a protobuf message.