Fix ListenerCoroutine shutdown check (use shutting_down_, drop unused running_)

server/server.cc

@@ -187,7 +187,7 @@ void Server::CloseHandler(ClientHandler *handler) {
 // the client.
 void Server::ListenerCoroutine(toolbelt::UnixSocket &listen_socket) {
   for (;;) {
-    if (!running_) {
+    if (shutting_down_) {
       // Keep this running until all other coroutines have completed.
       // This is to make sure that other coroutines that have publisher
       // and subscribers are able to connect to the server and delete them

server/server.h

@@ -223,7 +223,6 @@ class Server {
   std::string socket_name_;
   uint64_t session_id_;
   std::vector<std::unique_ptr<ClientHandler>> client_handlers_;
-  bool running_ = false;
   std::string server_id_;
   std::string hostname_;
   std::string interface_;