code: @Override
    public OperationFuture<Long> asyncDecr(String key, int by, long def,
      int exp) {
      return asyncMutate(Mutator.decr, key, by, def, exp);
    }
docs: Asynchronous decrement.
  
  @param key key to decrement
  @param by the amount to decrement the value by
  @param def the default value (if the counter does not exist)
  @param exp the expiration of this object
  @return a future with the decremented value, or -1 if the decrement failed.
  @throws IllegalStateException in the rare circumstance where queue is too
  full to accept any more requests