@Override
                public void run() {
                    logger.info("Shutting down the Executor for EurekaNotificationServerListUpdater");
                    try {
                        defaultServerListUpdateExecutor.shutdown();
                        Runtime.getRuntime().removeShutdownHook(shutdownThread);
                    } catch (Exception e) {
                        // this can happen in the middle of a real shutdown, and that's ok.
                    }
                }
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