Optimize `Llio`

See https://github.com/Incubaid/arakoon/commit/224dbc9fee05bfd4600d7c965dca61168a423a82.patch for a rationale.

Will launch CI first.
Another strategy might be to add a target string as parameter.  It's then up to the caller to re-allocate or reuse a target string, leaving it up to the caller to run the risk of races.