ISPN-4949 Split brain: data updated in degraded mode

https://issues.jboss.org/browse/ISPN-4949

Don't update the cache topology after a JGroups view update
until we know all the members are accessible.
Actually I think I made a mistake here, and should have used the degraded mode topology with the highest id instead. Once a partition enters degraded mode its CH won't change, but it could be that a partition managed to rebalance before losing another member and entering degraded mode.