code: @Override
    public OperationFuture<Boolean> prepend(String key, Object val) {
      return prepend(0, key, val, transcoder);
    }
docs: Prepend to an existing value in the cache.
  
  <p>
  Note that the return will be false any time a mutation has not occurred.
  </p>
  
  @param key the key to whose value will be prepended
  @param val the value to append
  @return a future indicating success
  @throws IllegalStateException in the rare circumstance where queue is too
  full to accept any more requests