message: core: attempt to fix connection race
  This is an attempt to prevent the race between the _connected flag and
  calling the on_change subscription.
diff: @@ -318,7 +318,7 @@ private:
  static constexpr double _HEARTBEAT_TIMEOUT_S = 3.0;
  std::mutex _connection_mutex{};
  - bool _connected{false};
  + std::atomic<bool> _connected{false};
  System::IsConnectedCallback _is_connected_callback{nullptr};
  void* _heartbeat_timeout_cookie = nullptr;
  