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
We could change this to WARN, but I wouldn't go lower.

Besides, we already log each segment for which we can't find an owner to transfer state from as an ERROR, I was thinking of replacing those with a single ERROR message here. (Though I haven't changed those other ERRORs to DEBUG yet.)