message: Remove state when receiving (forced) close
diff: @@ -291,8 +291,12 @@ struct downstream_msg_visitor {
  || std::is_same<T, downstream_msg::forced_close>::value) {
  inptr.reset();
  qs_ref.erase_later(dm.slots.receiver);
  + selfptr->remove_stream_manager(dm.slots);
  + if (mgr->done())
  + mgr->stop();
  + return intrusive::task_result::stop;
  }
  - if (mgr->done()) {
  + else if (mgr->done()) {
  CAF_LOG_DEBUG("path is done receiving and closes its manager");
  mgr->stop();
  selfptr->remove_stream_manager(dm.slots);
  