feat: Improve process termination logic in multiprocess manager

[abersheeran]

Summary

About https://github.com/encode/uvicorn/discussions/2369

In our cluster, we accidentally discovered the zombie process. We found the reason. Uvicorn's new process manager will JOIN child processes one by one after sending all exit signals. When the previous child process does not exit for a long time, the subsequent child processes cannot be JOIN.

I noticed that Uvicorn has an inherent shutdown timeout, which would be nice if we could use it with a multiprocessor.

The reason why we don't use terminate&join sequentially is to kill all processes faster, as mentioned in this PR https://github.com/encode/uvicorn/pull/2010

Checklist

• [x] 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.
• [x] I've updated the documentation accordingly.

[abersheeran]

I will add unit tests later, but I have no experience in designing a process that will hang, so if anyone can help, it would be greatly appreciated.