program exit, if registry use etcd, client must unregistry etcd before program exit

[Owen-Zhang]

如果使用etcd注册服务， 在程序退出前，应该主动unregistry etcd(取消服务注册)，不应该等etcd center 租约超时

publisher.go -> doKeepAlive() 

// 多处使用这种方式，导制退出没有取消服务注册
case <-p.quit.Done():
	return nil

// 可以增加一个服务反注册，取rpc注册服务更合理，同时client也可以更早的发现某个服务下线

[kevwan]

Check the code here.

https://github.com/zeromicro/go-zero/blob/896e1a2abb09254ec37952869e40b2eeee5f0200/core/discov/publisher.go#L149

[github-actions[bot]]

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