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.
not related to the PR:
don't understand this piece of code. in the line before, you are setting the ```mergedTopology = maxTopology``` and ```if (maxTopology != null)``` you are setting a new ```CacheTopology```. what is the pointing to set ```mergedTopology = maxTopology```?

BTW, the IDEA is telling me that the line 132 may produce NPE.