code: @Override
    public long decr(String key, long by) {
      return mutate(Mutator.decr, key, by, 0, -1);
    }
docs: Decrement the given key by the given value.
  
  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 value
  @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