private void Handle(ClusterActorDiscoveryMessage.UnregisterActor m)
        {
            _log.Debug($"UnregisterActor: Actor={m.Actor.Path}");

            // remove actor from _actorItems

            var index = _actorItems.FindIndex(a => a.Actor.Equals(m.Actor));

            if (index == -1)
            {
                return;
            }

            var tag = _actorItems[index].Tag;

            _actorItems.RemoveAt(index);
            UnwatchActor(m.Actor, 0);

            // tell monitors & other discovery actors that local actor down

            NotifyActorDownToMonitor(m.Actor, tag);
            foreach (var discoveryActor in _nodeMap.Keys)
            {
                discoveryActor.Tell(new ClusterActorDiscoveryMessage.ClusterActorDown(m.Actor));
            }
        }