sync: introduce NotifyGuard and refactor SetOnce to remove inner Mutex

[srxg]

This PR:

1. Introduces NotifyGuard, an internal type that wraps a lock on Notify’s waiter list and provides a method to wake all waiters while holding it.
2. removes the dedicated Mutex<()> from SetOnce by having it re-use the mutex already owned by Notify, via NotifyGuard.

Motivation

The goal is to remove the extra mutex inside SetOnce by having it reuse Notify's mutex. This avoids duplicate synchronisation and reduces memory footprint.

Solution

SetOnce now acquires the lock through Notify::lock, instead of holding its own mutex.

[ADD-SP]

I believe that eliminating the extra Mutex would improve both performance and code complexity.

it would be better for us to carefully consider how to achieve these benefits. The performance (including CPU and memory) penalty caused by the extra Mutex is not significant, which means that we should focus on reducing the code complexity.

The ideal case is that eliminating the extra Mutex and make people like me can understand the logic of critical code path in a few minutes.

[Darksonn]

Looks good to me.

[ADD-SP]

Thanks!