Topology-aware services need ability to control when service is added / removed from service map

In looking into solutions for MB-59571, it seems apparent that services need the ability to control when its services are added or removed from the advertised service map in the event of rebalance in & out.

One solution I pondered is adding two new task states for rebalances, in addition the existing states task-running and task-failed, two new optional statuses could be introduced:

• task-running-new-nodes-ready
• task-running-service-updates-ready

If a service returns task-running-new-nodes-ready, it would be an indication to ns_server that new KeepNodes are ready to receive requests from clients, and should be added to the service map.

If a service returns task-running-service-updates-ready, it would be an indication to ns_server that all updates (including that in task-running-new-nodes-ready) and EjectNodes should be removed from the service map. This is not unlike a normal clearing of the rebalance task which also indicates all the service map updates should be made, but in this case the rebalance is still underway and the service should not be terminated.