don't need to access backend when all filecache ready

[zyfjeff]

Version of nydus being used (nydusd --version)

Version: v2.2.2-14-g49359290
Git Commit: 4935929
Build Time: 2023-08-24T12:22:25.260520367Z
Profile: release
Rustc: rustc 1.66.1 (90743e729 2023-01-10)

Details about issue

After starting with filecache, the cache has already been downloaded and ready, in which case we should need to support offline startup nydusd, even if you can't access the backend

The current implementation always checks whether the corresponding blob exists at startup time.

        let reader = mgr
            .backend
            .get_reader(&blob_id)
            .map_err(|e| eio!(format!("failed to get reader for blob {}, {}", blob_id, e)))?;
        let blob_meta_reader = if is_separate_meta {
            mgr.backend.get_reader(&blob_meta_id).map_err(|e| {
                eio!(format!(
                    "failed to get reader for blob.meta {}, {}",
                    blob_id, e
                ))
            })?
        } else {
            reader.clone()
        };