message: Count up- and downstream messages for throughput
diff: @@ -274,7 +274,8 @@ operator()(size_t, upstream_queue&, mailbox_element& x) {
  auto& um = x.content().get_mutable_as<upstream_msg>(0);
  upstream_msg_visitor f{self, um};
  visit(f, um.content);
  - return intrusive::task_result::resume;
  + return ++handled_msgs < max_throughput ? intrusive::task_result::resume
  + : intrusive::task_result::stop_all;
  }
  namespace {
  @@ -333,7 +334,9 @@ operator()(size_t, downstream_queue& qs, stream_slot,
  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};
  - return visit(f, dm.content);
  + auto res = visit(f, dm.content);
  + return ++handled_msgs < max_throughput ? res
  + : intrusive::task_result::stop_all;
  }
  intrusive::task_result
  