message: Allow nesting request().await(), close
diff: @@ -423,13 +423,14 @@ invoke_message_result scheduled_actor::consume(mailbox_element& x) {
  // skip all messages until we receive the currently awaited response
  if (x.mid != pr.first)
  return im_skipped;
  - if (!pr.second(x.content())) {
  + auto f = std::move(pr.second);
  + awaited_responses_.pop_front();
  + if (!f(x.content())) {
  // try again with error if first attempt failed
  auto msg = make_message(make_error(sec::unexpected_response,
  x.move_content_to_message()));
  - pr.second(msg);
  + f(msg);
  }
  - awaited_responses_.pop_front();
  return im_success;
  }
  // handle multiplexed responses
  