message: set timeout for closing worker session
diff: @@ -1352,7 +1352,9 @@ Status MasterSession::DeleteWorkerSessions() {
  &workers[i].call_opts, &workers[i].request, &workers[i].response, cb);
  }
  - done.Wait();
  + if (!done.WaitFor(std::chrono::milliseconds(10000))) {
  + LOG(WARNING) << "Timeout for closing worker session";
  + }
  for (size_t i = 0; i < workers.size(); ++i) {
  status.Update(workers[i].status);
  }
  