PartitionHandling: consider less than numOwners partitions

If the following two conditions are simultaneously met during a split brain:
a) less than numOwners nodes leave
b) two clusters are formed having that size

Then both partitions would be considered as available. E.g. a cluster of 6 nodes with numOwners=4, with a split brain resulting in 3 and 3 nodes partitions.

We should add some logic to prevent this situation, e.g. by requiring the remaining partition to have at least numOwners members.