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

### Environment

* Dubbo version: 2.7.0
* Operating System version: mac
* Java version: 1.8

### Steps to reproduce this issue

1.  `AbstractClient#initConnectStatusCheckCommand` set break point at `  !isConnected()`.
2. `AbstractClient#close()` set break point here.    
3.  start client first, start provider and then shutdown provider gracefully , let method `AbstractClient.close()` be executed. Then select step `1` and trigger the thread to execute `!isConnected`, here will always try to connect the offline machine.

Pls. provide [GitHub address] to reproduce this issue.

### Expected Result

What do you expected from the above steps？

When the server goes offline, the client will not reconnect the offline machine.

### Actual Result

What actually happens?

The client reconnects the offline machine every 2 seconds and causes a large number of arp packets(because ip is recycled and not available now).
for (Invoker<T> invoker : new ArrayList<Invoker<T>>(localUrlInvokerMap.values())) {
                if (invoker.isAvailable()) {
                    return true;
                }
            }
        }
        return false;
    }
    public void buildRouterChain(URL url) {
        this.setRouterChain(RouterChain.buildChain(url));
    }
    /**
     * Haomin: added for test purpose
     */
    public Map<String, Invoker<T>> getUrlInvokerMap() {
        return urlInvokerMap;
    }
    public List<Invoker<T>> getInvokers() {
        return invokers;
    }
    private boolean isValidCategory(URL url) {
        String category = url.getParameter(CATEGORY_KEY, DEFAULT_CATEGORY);
        if ((ROUTERS_CATEGORY.equals(category) || ROUTE_PROTOCOL.equals(url.getProtocol())) ||
                PROVIDERS_CATEGORY.equals(category) ||
                CONFIGURATORS_CATEGORY.equals(category) || DYNAMIC_CONFIGURATORS_CATEGORY.equals(category) ||
                APP_DYNAMIC_CONFIGURATORS_CATEGORY.equals(category)) {
            return true;
        }
        logger.warn("Unsupported category " + category + " in notified url: " + url + " from registry " +
                getUrl().getAddress() + " to consumer " + NetUtils.getLocalHost());