Add negative metadata cache ttl

[notoriaga]

Adds a new CLI argument --negative-cache-ttl that lets you set the TTL for negative metadata entries separately from --metadata-ttl. My use case is a write once read many bucket. Objects do not get deleted from this bucket, and new objects are added every few minutes. I'd like to be able to set --metadata-ttl indefinite and --negative-cache-ttl 60 to effectively utilize the caching while still being able to pick up new objects. There is an open issue for this here - #831

Does this change impact existing behavior?

No, if --negative-cache-ttl is omitted the existing behavior is maintained (use --metadata-ttl or the default file_ttl).

Does this change need a changelog entry? Does it require a version change?

Because this is a new feature I believe it would require both.

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[passaro]

Hi @notoriaga, thank you for opening this pull request!

I've got a few suggestions, but the next step would be to add a couple of tests. See existing tests for the negative cache, e.g. test_negative_lookup_with_caching in superblock.rs and lookup_with_negative_cache in lookup_test.rs. Happy to help if you need more guidance.

[passaro]

Just setting the TTL here may not be enough. We use the same serve_lookup_from_cache flag both for the "positive" cache and the negative cache (here, in particular). I suspect we need to introduce a separate flag and ensure it is set to false on Minimal here.