ISPN-4574 PartitionHandling: consider less than numOwners partitions

https://issues.jboss.org/browse/ISPN-4574

* Merge PartitionHandlingStrategy with RebalancePolicy -> AvailabilityStrategy
* Add a stable topology with the last topology that didn't need rebalancing
* NumOwnersNodeStopInSequenceTest - stop nodes gracefully instead of abruptly
Sounds good.