ISPN-263 Handle cluster partitions

https://issues.jboss.org/browse/ISPN-263

I added one more commit with some test fixes.
I've created https://issues.jboss.org/browse/ISPN-4624 to allow plugging in an implementation (although we might stick with a boolean/enum for 7.0).

This is how we recognize partitions: "We assume a partition happens when *numOwners* or more nodes disappear at the same time." But I also think that distinguishing between 2 owners leaving in the same JGroups view or in 2 consecutive views is a bit arbitrary - I would rather have a single degraded mode covering both cases.