message: improve the accuracy of ZIncrbyCmd()
diff: @@ -145,9 +145,10 @@ void ZIncrbyCmd::Do() {
  double score = 0;
  rocksdb::Status s = g_pika_server->db()->ZIncrby(key_, member_, by_, &score);
  if (s.ok()) {
  - std::string new_value = std::to_string(score);
  - res_.AppendStringLen(new_value.size());
  - res_.AppendContent(new_value);
  + char buf[32];
  + int64_t len = slash::d2string(buf, sizeof(buf), score);
  + res_.AppendStringLen(len);
  + res_.AppendContent(buf);
  } else {
  res_.SetRes(CmdRes::kErrOther, s.ToString());
  }
  