perf: user-level IO scheduler

[petermattis]

ScyllaDB claims their user-level IO scheduler provides better latencies under any circumstance.

Jira issue: PEBBLE-189

[petermattis]

See also https://www.usenix.org/system/files/atc19-balmau.pdf.

[github-actions]

We have marked this issue as stale because it has been inactive for
18 months. If this issue is still relevant, removing the stale label
or adding a comment will keep it active. Otherwise, we'll close it
in 10 days to keep the issue queue tidy. Thank you for your
contribution to Pebble!

[jbowens]

https://www.scylladb.com/2022/08/03/implementing-a-new-io-scheduler-algorithm-for-mixed-read-write-workloads/