PartitionHandling: remove unavailable mode

The Unavailable mode name is misleading, because some keys are available, just like in Degraded mode.

The only difference between Degraded and Unavailable is that with Degraded the cluster might recover without manual intervention. The administrator still has to know a lot more details in order to decide whether manual intervention is needed or not. So it would be less confusing if gracefully shutting down numOwners nodes in quick succession would leave the cache in Degraded mode instead of Unavailable.

Instead of removing the Unavailable mode completely, we could also change it to deny access to all the keys and allow the administrator to use it. E.g. if we had an operation to dump the cache into a shared store and another to load the cache from a shared store, the administrator could force the cache into Unavailable mode while dumping/loading the cache.