Fix timeout on async server shutdown

Server was continuing to make requests for new calls forever, which were
starving out the shutdown sequence. Change order and win.
diff --git a/test/cpp/qps/server_async.cc b/test/cpp/qps/server_async.cc
index 1234542..1eddb1d 100644
--- a/test/cpp/qps/server_async.cc
+++ b/test/cpp/qps/server_async.cc
@@ -131,10 +131,10 @@
     }
   }
   ~AsyncQpsServerTest() {
-    server_->Shutdown();
     for (auto ss = shutdown_state_.begin(); ss != shutdown_state_.end(); ++ss) {
       (*ss)->set_shutdown();
     }
+    server_->Shutdown();
     for (auto thr = threads_.begin(); thr != threads_.end(); thr++) {
       thr->join();
     }