message: Respect shutdown mode in I/O servants
diff: @@ -92,7 +92,8 @@ protected:
  invoke_mailbox_element_impl(ctx, value_);
  // only consume an activity token if actor did not produce them now
  if (prev && activity_tokens_ && --(*activity_tokens_) == 0) {
  - if (this->parent()->getf(abstract_actor::is_terminated_flag))
  + if (this->parent()->getf(abstract_actor::is_shutting_down_flag
  + | abstract_actor::is_terminated_flag))
  return false;
  // tell broker it entered passive mode, this can result in
  // producing, why we check the condition again afterwards
  