message: rafs: unnecessary to wait for 2 seconds before staring `prefetch_all`
  User specifeid prefetch files are already queued before `prefetch_all`
  prefetch IO. So no need to sleep for 2 seconds. There is no contend between
  `hinted` and `prefetch_all`.
diff: @@ -319,10 +319,6 @@ impl Rafs {
  if prefetch_all {
  let root = vec![RAFS_ROOT_INODE];
  - // Sleeping for 2 seconds is indeed a trick to prefetch the entire rootfs.
  - // FIXME: As nydus can't give different policies different priorities, this is a
  - // workaround. Remove the sleeping when IO priority mechanism is ready.
  - sleep(Duration::from_secs(2));
  sb.prefetch_hint_files(&mut reader, Some(root), &|mut desc| {
  device.prefetch(&mut desc).unwrap_or_else(|e| {
  warn!("Prefetch error, {:?}", e);
  