code: @Override
    public long incr(String key, int by) {
      return mutate(Mutator.incr, key, by, 0, -1);
    }
docs: Increment the given key by the given amount.
  
  Due to the way the memcached server operates on items, incremented and
  decremented items will be returned as Strings with any operations that
  return a value.
  
  @param key the key
  @param by the amount to increment
  @return the new value (-1 if the key doesn't exist)
  @throws OperationTimeoutException if the global operation timeout is
  exceeded
  @throws IllegalStateException in the rare circumstance where queue is too
  full to accept any more requests