message: fix warning: comparison of integer expressions of different signedness
diff: @@ -274,7 +274,7 @@ private:
  inline std::vector<status> wait_all(std::vector<request>& req)
  {
  std::vector<status> stat;
  - for (auto i = 0; i < req.size(); ++i) {
  + for (std::size_t i = 0; i < req.size(); ++i) {
  stat.emplace_back(req[i].wait());
  }
  return stat;
  