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.
Yeah, it is a bit confusing... the if condition was more complex before, and then it made sense to initialize with `maxTopology`. I'm changing the initialization value to `null`.

And IntelliJ is right, we could get a NPE if there are only joiners without a CH. I've rewritten that bit as well.