ISPN-263 Handle cluster partitions

https://issues.jboss.org/browse/ISPN-263

I added one more commit with some test fixes.
Let's say we have a cluster with 4 nodes, numOwners = 2, and there is a 3 to 1 split.
The node that is by itself should enter degraded mode, but the others should rebalance stay in available mode. If the 1-node partition would allow reads, the user could see stale entries.