优雅停机时无法调用dubbo协议下的unexport方法

[jianpeng957]

• [ ] I have searched the issues of this repository and believe that this is not a duplicate.
• [ ] I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: 2.7.6
• Operating System version: MACOS 10.15.X
• Java version: 1.8.0_131

Steps to reproduce this issue

1. 实现ExporterListener接口，并重载unexport方法
2. 实现dubbo协议的接口
3. 优雅停机，发现unexport方法并未被调用

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

Expected Result

What do you expected from the above steps？

Actual Result

What actually happens?

If there is an exception, please attach the exception trace:

Just put your stack trace here!

考虑的解决方式：

1. 发现代码存在疑点，如图：org..apache.dubbo.registry.integration.RegistryProtocol.java:743
2. 删除sleep代码或者减少sleep时间，应该能避免这个问题
3. 并且org.apache.dubbo.common.config.ConfigurationUtils#getServerShutdownTimeout标记了//FIXME，说明这是遗留问题？

[manzhizhen]

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