message: Remove monitoring when workers leave a pool
diff: @@ -183,6 +183,9 @@ bool actor_pool::filter(upgrade_lock<detail::shared_spinlock>& guard,
  auto last = workers_.end();
  auto i = std::find(workers_.begin(), last, what);
  if (i != last) {
  + default_attachable::observe_token tk{address(),
  + default_attachable::monitor};
  + what->detach(tk);
  workers_.erase(i);
  }
  return true;
  