Server transport and executor metrics are not unregistered on stop

Server transport and executor are registered in JMX with CacheManagerJmxRegistration.registerExternalMBean(), which also registers them in the metrics registry.

But they're unregistered with JmxUtil.unregisterMBean(), which doesn't unregister them from the metrics registry.

We really need to remove JmxUtil and only use the xJmxRegistration components everywhere except the client.