Forced termination of `aiotools.fork` is not working on macOS

### What Operating System(s) are you seeing this problem on?

macOS 15.0.1

### Backend.AI version

main

### Describe the bug

After sending a SIGINT signal, if the process does not terminate within a specified period, `aiotool.afork()` sends a SIGKILL signal to forcibly terminate the process.

This mechanism works without issues on Linux, where it operates via `PidfdChildProcess`, but it is not functioning on macOS.

Debugging results suggest that the issue may be related to the timing of setting `self._terminated` to True.

This may be an upstream issue with `aiotools`, but as it is also affecting the behavior of Backend.AI, I am creating an issue for it.

### To Reproduce

1. Start the Backend.AI manager server.
1. Shut down the Redis container.
1. Press Ctrl + C to send the SIGINT signal.
1. Although below log appear, the SIGKILL signal is not actually sent. So, the manager process also does not terminate.
{code}
2024-11-06 14:26:38.036 WARNING aiotools.server [10753] Timeout during waiting for child processes; killing all

{code}

> [!NOTE]This example uses the manager component, but the issue can be reproduced with other components as well.

### Expected Behavior

The SIGKILL signal is sent successfully, and the manager process terminates.(Behavior on Linux.)

### Anything else?

None

JIRA Issue: BA-65