AIO: Switching from thread pools to io_uring

[Florian Weimer]

* Dhruv McElwaine:

> Currently, glibc's aio implementation relies on a thread pool to provide
> parralelism. With io_uring turning 6 tomorrow, I have started work on a
> reimplementation of aio based on io_uring. Would it be worth trying to
> finish it off and merge it into glic proper?

If it can produce the required semantics, that would be really nice.
(We probably need more AIO tests, possibly using the FUSE test framework.)