[io] Force async buffered read SQE submission
[io] Force async buffered read SQE submission

By enabling the IOSQE_ASYNC flag, we force io-uring to maximize
read throughput. The effects of this flag appear to be machine
and kernel-dependent. A benchmark run on one development setup
showed a 0.9GB/s throughput increase (from 2.4GB/s to 3.3GB/s).
However, benchmark performance on another setup was unchanged.

olifre/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root