use AtomicWaker instead of Mutex<Option<Waker>>

[cfeitong]

futures crate provides a useful synchronization primitive AtomicWaker. I think it is a better choice over Mutex<Option<Waker>>.

[ibraheemdev]

Sorry for the late review, I don't want to pull in the entire futures crate for this, could you update the code to depend on the atomic-waker crate instead?

[chen-hongzhi]

@ibraheemdev friendly ping.

[ibraheemdev]

Sorry, I didn't get notified that you made the change. Just one small nit before merging.

[ibraheemdev]

Thanks!