message: Emit receive entries for up- and downstream msgs
diff: @@ -269,6 +269,8 @@ struct upstream_msg_visitor {
  intrusive::task_result scheduled_actor::mailbox_visitor::
  operator()(size_t, upstream_queue&, mailbox_element& x) {
  CAF_ASSERT(x.content().type_token() == make_type_token<upstream_msg>());
  + self->current_mailbox_element(&x);
  + CAF_LOG_RECEIVE_EVENT((&x));
  auto& um = x.content().get_mutable_as<upstream_msg>(0);
  upstream_msg_visitor f{self, um};
  visit(f, um.content);
  @@ -326,6 +328,8 @@ operator()(size_t, downstream_queue& qs, stream_slot,
  policy::downstream_messages::nested_queue_type& q,
  mailbox_element& x) {
  CAF_LOG_TRACE(CAF_ARG(x) << CAF_ARG(handled_msgs));
  + self->current_mailbox_element(&x);
  + CAF_LOG_RECEIVE_EVENT((&x));
  CAF_ASSERT(x.content().type_token() == make_type_token<downstream_msg>());
  auto& dm = x.content().get_mutable_as<downstream_msg>(0);
  downstream_msg_visitor f{self, qs, q, dm};
  