Add io_uring backend

[doublej472]

This PR adds a new backend using io_uring with the liburing library, supporting both file and block device backends.

I've been running this at home for a few months without issue, and figured it's about time I open a PR to bring it into upstream.

This backend supports WRITE, READ, SYNCHRONIZE_CACHE, and UNMAP operations

UNMAP is handled a bit special, since we can receive multiple in a single request, and need to perform a different action depending on if the backing device is a block device or regular file.

• For regular files, we submit a group of linked fallocate SQE's with FALLOC_FL_PUNCH_HOLE and FALLOC_FL_KEEP_SIZE.
• For devices, we need to perform synchonous BLKDISCARD requests. Once https://patchwork.kernel.org/project/io-uring/patch/a672ddb6-a141-47a5-b7f4-f992b4dcbb88@kernel.dk/ hits stable we can perform DISCARD requests in io_uring, but we still need to wait for that to make it into the stable kernel, and for liburing to be updated.