chunked: add fuzzing tests for the cache

[giuseppe]

and fix some issues found with the new tests.

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[cgwalters]

I was unaware of this whole subsystem until now. This is the thing keeping indicies of chunks that are used for inside-a-file partial pulls?

The changes seem sane superficially.

[giuseppe]

I was unaware of this whole subsystem until now. This is the thing keeping indicies of chunks that are used for inside-a-file partial pulls?

yes, this is needed to avoid parsing the json metadata files each time we lookup for a digest. In the same cache file we store the files payload digest, the chunks digest and the hard link fingerprint. The hard link fingerprint a digest that is used to find files that can be used for hard links deduplication (payload digest + UID + GID + mode + xattrs).

[rhatdan]

/lgtm