Optimize `Llio`

See https://github.com/Incubaid/arakoon/commit/224dbc9fee05bfd4600d7c965dca61168a423a82.patch for a rationale.

Will launch CI first.
@mark-christiaens made the same comment yesterday. Assuming non-preempted 'single-threaded' execution this might be true, but I'm reluctant to rely on such things... If ever OCaml would support a parallel runtime, this could introduce hard-to-pinpoint bugs.