Add Submitter::squeue_wakeup()

[utkarshgupta137]

I'm using io-uring with sqpoll enabled with a very large idle timeout because I want to minimize the latency of processing data. But I still want to check if it has gone idle from time to time. Currently this requires calling io_uring.submission().need_wakeup(), which causes an extra Acquire load on the squeue ring. Add a function that just wakes it up, without trying to return how many events were pending. Alternatively, we could also add need_wakeup to IoUring, similar to issue #18.