Replace global value of Stats by a DLS.key

[lyrm]

This patch allows multiple domains to compute their own stats and merge them with the new function merge.

Examples:

let work barrier id =
  Atomic.decr barrier;
  while Atomic.get barrier <> 0 do
    Domain.cpu_relax ()
  done;
  add_read (10 * id);
  add_write (10 * id);
  get ()

let () =
  let ndomain = 5 in
  let barrier = Atomic.make ndomain in
  let domains =
    Array.init ndomain (fun id -> Domain.spawn (fun () -> work barrier id))
  in
  let stats = Array.map (fun d -> Domain.join d) domains in
  Array.iteri
    (fun id s ->
      Printf.printf "Domain %d: read %d write %d\n" id s.bytes_read
        s.bytes_written)
    stats;
  let total = Array.fold_left merge (fresh_stats ()) stats in
  Printf.printf "Total: read %d write %d\n" total.bytes_read total.bytes_written

returns

Domain 0: read 0 write 0
Domain 1: read 10 write 10
Domain 2: read 20 write 20
Domain 3: read 30 write 30
Domain 4: read 40 write 40
Total: read 100 write 100

[clecat]

LGTM, thanks a lot for your work.
DLS seems to be a good option to resolve the multicore issues of Index stats.