ClusteredCacheFactory.isSeniorClusterMember() incorrectly returns false

[GregDThomas]

There is a small window of in-opportunity where ClusteredCacheFactory.isSeniorClusterMember() can return false when the node is actually the senior member.

Although in theory this can only last a few CPU cycles, in practice because these CPU cycles are also used to fire the ClusterEventListener#joinedCluster() event it can be very misleading.