chunked: implement lookaside cache

avoid parsing each json TOC file for the layers in the local storage,
but attempt to create a lookaside cache in a custom format faster to
load (and potentially be mmap'able).

The same cache is used to lookup files, chunks and candidates for
deduplication with hard links.

There are 3 kind of digests stored:

- digest(file.payload))
- digest(digest(file.payload) + file.UID + file.GID + file.mode + file.xattrs)
- digest(i) for each i in chunks(file payload)

Signed-off-by: Giuseppe Scrivano <[email protected]>