message: wait for m_callback_queue to be empty on stop_observing()
diff: @@ -112,6 +112,10 @@ void NetworkReachabilityObserver::stop_observing()
  {
  SCNetworkReachabilitySetDispatchQueue(m_reachability_ref.get(), nullptr);
  SCNetworkReachabilitySetCallback(m_reachability_ref.get(), nullptr, nullptr);
  +
  + // Wait for all previously-enqueued blocks to execute to guarantee that
  + // no callback will be called after returning from this method
  + dispatch_sync(m_callback_queue, ^{});
  }
  void NetworkReachabilityObserver::reachability_changed()
  