fix: make sure always execute the shutdown logic, even if server rece…

[xianml]

make sure always execute the shutdown logic, even if server received a signal during startup
before: can not shutdown server when i try to shutdown a server during startup

after:

Summary

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Kludex]

This doesn't follow the ASGI specification. There are related issues, please search them on https://github.com/encode/uvicorn/issues.

[xianml]

@Kludex actually it is not the same as #1301 etc.
no matter or what, the initialization will run to complete according to ASGI specification. But what i want to change is uvicorn always need to run server.close to shutdown the server and close the socket. But currently, some of the process will not got the chance to execute the shutdown logic.

Please help review again. I am not to break the ASGI specification, just want to make sure the shutdown logic is comprehensive.