private <R, A> void addCallback(
      ListenableFuture<R> future, CompletionHandler<R, ? super A> handler, @Nullable A attachment) {
    future.addListener(new CompletionHandlerCallback<>(future, handler, attachment), executor);
  }
{@link AsynchronousFileChannel} implementation that delegates to a {@link JimfsFileChannel}.

@author Colin Decker
jimfs/src/main/java/com/google/common/jimfs/JimfsAsynchronousFileChannel.java