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
I suppose there's no reason not to allow access to keys that are fully owned by the live members. 

We could also allow at least read access for partially-owned keys, but for now I'm going to keep it simple and use the same rules for unavailable and degraded modes.