message: core: use queue for subsciption callback
  I found some segfaults around this and thought it was a good idea.
diff: @@ -605,7 +605,8 @@ void MavsdkImpl::subscribe_on_new_system(const Mavsdk::NewSystemCallback& callba
  _new_system_callback = callback;
  if (_new_system_callback != nullptr && is_any_system_connected()) {
  - _new_system_callback();
  + auto temp_callback = _new_system_callback;
  + call_user_callback([temp_callback]() { temp_callback(); });
  }
  }
  