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 had numOwners=3, and 3 nodes leave at once. The cluster is correctly marked as degraded (as I see this patch doesn't look at lost segments yet, only at numbers of nodes).
What kind of event is needed for the degraded mode to be withdrawn?
I'm going to assume that it's not enough to start a new node, which counts as +1, but that I actually need a node to return and having knowledge about the missing segment, at the matching view.

Two things missing:
 - document the healing protocol
 - consider what happens if the missing group of servers isn't also degraded, and happens to makes progress: is the smaller group going to be able to heal?