IORing: support SQPOLL

I would like to use iouring with SQPOLL to avoid unnecessary syscalls.

 I found this comment in the code and wonder if there are any plans of supporting it...
```
    // Ring always needs enter right now;
    // TODO: support SQPOLL here
    while true {
        let ret = io_uring_enter(ringDescriptor, numEvents, minCompletions, flags, nil)

```