Async I/O support

[alysenko-polytech]

From https://github.com/elastio/scalez/issues/488:

We recently updated the Elastio AMIs to have kernel 5.15, which has all of the latest io_uring features. RocksDB supports async I/O using io_uring, which potentially is more performant under heavy load. Experiment with enabling async I/O in our workloads. See this blog post for a write-up of async I/O in Rocks.
UPDATE: Sadly this is not going to work yet, async MultiGet requires that RocksDB be linked with the folly C++ library and I was unable to get that radioactive dumpster fire of C++ shit to compile reliably, so while all of the #defines are in place Rocks won't actually do async MultiGet in the latest version of roxide.

Compiling and linking folly may be challenging, but it's still worth consideration to finish it and add async I/O support to boost RocksDB performance.