- [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).
Channel channel = getChannel();
        if (channel == null) {
            return false;
        }
        return channel.hasAttribute(key);
    }
    @Override
    public void send(Object message, boolean sent) throws RemotingException {
        if (send_reconnect && !isConnected()) {
            connect();
        }
        Channel channel = getChannel();
        
        if (channel == null || !channel.isConnected()) {
            throw new RemotingException(this, "message can not send, because channel is closed . url:" + getUrl());
        }
        channel.send(message, sent);
    }
    protected void connect() throws RemotingException {
        connectLock.lock();
        try {
            if (isConnected()) {
                return;
            }
            initConnectStatusCheckCommand();
            doConnect();
            if (!isConnected()) {
                throw new RemotingException(this, "Failed connect to server " + getRemoteAddress() + " from " + getClass().getSimpleName() + " "
                        + NetUtils.getLocalHost() + " using dubbo version " + Version.getVersion()
                        + ", cause: Connect wait timeout: " + getConnectTimeout() + "ms.");