pub(crate) unsafe fn start_poll(&self) {
        self.status.store(POLLING, SeqCst);
    }
Alert the mutex that polling is about to begin, clearing any accumulated
re-poll requests.

# Safety

Callable only from the `POLLING`/`REPOLL` states, i.e. between
successful calls to `notify` and `wait`/`complete`.
futures-executor/src/unpark_mutex.rs