@Override
    public long getDurationSinceLastUpdateMs() {
        return System.currentTimeMillis() - lastUpdated.get();
    }
A server list updater for the {@link com.netflix.loadbalancer.DynamicServerListLoadBalancer} that
utilizes eureka's event listener to trigger LB cache updates.

Note that when a cache refreshed notification is received, the actual update on the serverList is
done on a separate scheduler as the notification is delivered on an eurekaClient thread.

@author David Liu
ribbon-eureka/src/main/java/com/netflix/niws/loadbalancer/EurekaNotificationServerListUpdater.java