Call to GetClusterMembers RPC throws NPE

When calling the GetClusterMembers RPC method, StatsCollector throws an NPE (see attached stack trace).

This is due to a change in the exposed MBeans which StatsCollector relied on to get the members of the cluster. The relevant MBean is no longer present, so we get an NPE when we try to get a field from the resulting jolokia JSON object.

The GetClusterMembers RPC should really use the akka MBean to determine cluster members instead of the hacky way it was done in StatsCollector before.