Fix BatMutex.DebugMutex.id is always 0.

Perhaps DebugMutex.unique intends to be closure, but it is constant now.
```ocaml
open Batteries
module Mutex =  BatMutex.DebugMutex

let _ =
  let m = Mutex.create () in
  let m2 = Mutex.create () in
  Mutex.lock m;
  Mutex.lock m2;
  Mutex.unlock m2;
  Mutex.unlock m;
  ()
```