Fix benchmark shutdown

test/cpp/qps/server_async.cc

@@ -132,8 +132,11 @@ class AsyncQpsServerTest : public Server {
       (*ss)->shutdown = true;
     }
     // TODO (vpai): Remove this deadline and allow Shutdown to finish properly
-    auto deadline = std::chrono::system_clock::now() + std::chrono::seconds(3);
-    server_->Shutdown(deadline);
+    std::thread shutdown_thread([this]() {
+      auto deadline =
+          std::chrono::system_clock::now() + std::chrono::seconds(3);
+      server_->Shutdown(deadline);
+    });
     for (auto cq = srv_cqs_.begin(); cq != srv_cqs_.end(); ++cq) {
       (*cq)->Shutdown();
     }
@@ -146,6 +149,7 @@ class AsyncQpsServerTest : public Server {
       while ((*cq)->Next(&got_tag, &ok))
         ;
     }
+    shutdown_thread.join();
   }
 
  private: