code: @Override
    public OperationFuture<Long> asyncIncr(String key, long by, long def) {
      return asyncMutate(Mutator.incr, key, by, def, 0);
    }
docs: Asychronous increment.
  
  @param key key to increment
  @param by the amount to increment the value by
  @param def the default value (if the counter does not exist)
  @return a future with the incremented value, or -1 if the increment failed.
  @throws IllegalStateException in the rare circumstance where queue is too
  full to accept any more requests