* Example usage:
   *
   * <pre>{@code
   * ListenableFuture<RowKey> rowKeyFuture = indexService.lookUp(query);
   * ListenableFuture<QueryResult> queryFuture =
   *     transformAsync(rowKeyFuture, dataService::readFuture, executor);
   * }</pre>
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the warnings the {@link MoreExecutors#directExecutor} documentation.

   * Example usage:
   *
   * <pre>{@code
   * ListenableFuture<RowKey> rowKeyFuture = indexService.lookUp(query);
   * ListenableFuture<QueryResult> queryFuture =
   *     transformAsync(rowKeyFuture, dataService::readFuture, executor);
   * }</pre>
   *
   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
   * the warnings the {@link MoreExecutors#directExecutor} documentation.