message: replace loop with ranged-for from worker's get_status
diff: @@ -34,8 +34,8 @@ void Worker::GetStatusAsync(const GetStatusRequest* request,
  std::vector<DeviceAttributes> devices;
  dm->ListDeviceAttributes(&devices);
  response->mutable_device_attributes()->Reserve(devices.size());
  - for (size_t i = 0; i < devices.size(); ++i) {
  - response->add_device_attributes()->Swap(&devices[i]);
  + for (auto& d : devices) {
  + response->add_device_attributes()->Swap(&d);
  }
  done(Status::OK());
  }
  