message: add lvalue ref for auto deduction, and aviod to copy object
diff: @@ -27,7 +27,7 @@ class WorkerFreeListCache : public WorkerCacheInterface {
  : wrapped_(std::move(w)) {}
  ~WorkerFreeListCache() final {
  - for (auto p : workers_) {
  + for (auto& p : workers_) {
  wrapped_->ReleaseWorker(p.first, p.second.worker);
  }
  }
  