message: specify DISPATCH_QUEUE_SERIAL explicitly
diff: @@ -58,7 +58,7 @@ static void reachability_callback(SCNetworkReachabilityRef, SCNetworkReachabilit
  NetworkReachabilityObserver::NetworkReachabilityObserver(util::Optional<std::string> hostname,
  std::function<void (const NetworkReachabilityStatus)> handler)
  -: m_callback_queue(dispatch_queue_create("io.realm.sync.reachability", nullptr))
  +: m_callback_queue(dispatch_queue_create("io.realm.sync.reachability", DISPATCH_QUEUE_SERIAL))
  , m_reachability_callback([=]() { reachability_changed(); })
  , m_change_handler(std::move(handler))
  {
  