message: Fix cleanup with middleman_detach_multiplexer
diff: @@ -294,9 +294,14 @@ void middleman::stop() {
  }
  }
  });
  + if (system_.config().middleman_detach_multiplexer) {
  backend_supervisor_.reset();
  if (thread_.joinable())
  thread_.join();
  + } else {
  + while (backend().try_run_once())
  + ; // nop
  + }
  hooks_.clear();
  named_brokers_.clear();
  scoped_actor self{system(), true};
  