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- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.14、3.0.4

### Steps to reproduce this issue

1. start dubbo admin (nacos registry)

The ```EventListener``` instance has created countless times by the scheduled task(```scheduledExecutorService```) and cached in the ```InstancesChangeNotifier#listenerMap```(Nacos)  , until the memory is full.

- NacosRegistry
```
private void subscribeEventListener(String serviceName, final URL url, final NotifyListener listener)
        throws NacosException {
        EventListener eventListener = new RegistryChildListenerImpl(serviceName, url, listener);
        namingService.subscribe(serviceName,
            getUrl().getGroup(Constants.DEFAULT_GROUP),
            eventListener);
}
```

- InstancesChangeNotifier
```
private final Map<String, ConcurrentHashSet<EventListener>> listenerMap = new ConcurrentHashMap<String, ConcurrentHashSet<EventListener>>();

public void registerListener(String serviceName, String clusters, EventListener listener) {
        String key = ServiceInfo.getKey(serviceName, clusters);
        ConcurrentHashSet<EventListener> eventListeners = listenerMap.get(key);
        if (eventListeners == null) {
            synchronized (lock) {
                eventListeners = listenerMap.get(key);
                if (eventListeners == null) {
                    eventListeners = new ConcurrentHashSet<EventListener>();
                    listenerMap.put(key, eventListeners);
                }
            }
        }
        eventListeners.add(listener);
}
```
The ```namingService.subscribe```  is finally called to the ```InstancesChangeNotifier#registerListener``` , so 
eventListener cached in  eventListeners.

![image](https://user-images.githubusercontent.com/5037807/140080041-f617bda2-0f48-4669-8e3d-2184a6eb9a75.png)



* The separator for service name
     * Change a constant to be configurable, it's designed for Windows file name that is compatible with old
     * Nacos binary release(< 0.6.1)
     */
    private static final String SERVICE_NAME_SEPARATOR = System.getProperty("nacos.service.name.separator", ":");
    /**
     * The pagination size of query for Nacos service names(only for Dubbo-OPS)
     */
    private static final int PAGINATION_SIZE = Integer.getInteger("nacos.service.names.pagination.size", 100);
    /**
     * The interval in second of lookup Nacos service names(only for Dubbo-OPS)
     */
    private static final long LOOKUP_INTERVAL = Long.getLong("nacos.service.names.lookup.interval", 30);
    private static final Logger logger = LoggerFactory.getLogger(NacosRegistry.class);
    private final NacosNamingServiceWrapper namingService;
    /**
     * {@link ScheduledExecutorService} lookup Nacos service names(only for Dubbo-OPS)
     */
    private volatile ScheduledExecutorService scheduledExecutorService;
    public NacosRegistry(URL url, NacosNamingServiceWrapper namingService) {
        super(url);
        this.namingService = namingService;
    }
    @Override
    public boolean isAvailable() {
        return UP.equals(namingService.getServerStatus());
    }
    @Override
    public List<URL> lookup(final URL url) {
        if (url == null) {