QueryHandle::next_row_async never schedules a waking task

[teh-cmc]

See:

    pub fn next_row_async(
        &self,
    ) -> impl std::future::Future<Output = Option<Vec<Box<dyn ArrowArray>>>> {
        let res: Option<Option<_>> = self
            .engine
            .try_with(|store, cache| self._next_row(store, cache));

        std::future::poll_fn(move |_cx| match &res {
            Some(row) => std::task::Poll::Ready(row.clone()),
            None => std::task::Poll::Pending,
        })
    }

The pending case just casually goes to sleep without ever scheduling a waking task. This will invariably lead to the async tasks deadlocking.

This needs to queue some task e.g. on the rayon threadpool in order to waker.wake() when the synchronous lock becomes available once again.
No need for anything particularly fancy, QueryHandle is the least contended thing in the world.