Optimize `Llio`

See https://github.com/Incubaid/arakoon/commit/224dbc9fee05bfd4600d7c965dca61168a423a82.patch for a rationale.

Will launch CI first.
There already  something that's _passed in_ : the (buffer : Buffer.t). So if you really want to fiddle it's better to replace the type of the buffer. Then again, it might be better to review the protocol to reduce the number of string copies altogether. Without a typical use case it's hard to see what exactly the value is of such an optimization effort.