reactor: replace ATOMIC_USIZE_INIT with AtomicUsize::new(0) (#889)
reactor: replace ATOMIC_USIZE_INIT with AtomicUsize::new(0) (#889)

ATOMIC_BOOL_INIT is deprecated since 1.34 because the const fn
AtomicUsize::new is now preferred. As deny(warnings) is set,
tokio fails to build on latest nightly. This will fix it.

Signed-off-by: Yilin Chen <9f6bed1fd69d0e15c5d9fbf33319f3d0f8cbc703@gmail.com>
tokio-rs/tokio,tokio-rs/tokio