Fix race condition that causes TimerThread to hang during shutdown

[gitccl]

What problem does this PR solve?

Problem Summary:

During shutdown, stop_and_join() sets _stop = true, sets _nearest_run_time = 0, and calls futex_wake_private() to wake up the timer thread. However, due to a race condition, the timer thread may:

1. See _stop == false and enter the loop.
2. Block on acquiring _mutex while stop_and_join() holds it.
3. After stop_and_join() releases the mutex, the timer thread acquires it and sets _nearest_run_time = int64_max, overwriting the earlier _nearest_run_time = 0.
4. Miss the wake-up signal because at the time stop_and_join() called futex_wake_private(), the timer thread had not yet entered futex wait.

As a result, after finishing all remaining tasks, the timer thread may enter futex wait with no further wake-up, causing the program to hang on pthread_join.

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[wwbmmm]

LGTM

[chenBright]

LGTM