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.
This is a little trickier, as the comment says we can't enter degraded mode without having a stable topology and losing some of the stable topology members. I guess it could happen if the user entered degraded mode with a JMX operation.