ISPN-263 Handle cluster partitions

https://issues.jboss.org/browse/ISPN-263

I added one more commit with some test fixes.
Yes, I think it is intended to be public API (or rather SPI), but we're not exposing it anywhere yet (and I don't know if we'll do it for 7.0.0.Final either). So I'd rather keep it in the impl package for now.