Add a QueuedFile object

[msm-cert]

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A smaller version of #432.

This PR creates a "core" (database object) of the index queue that will be managed by ursadb. This will let us queue files for indexing, and index them with available plugins.

This PR doesn't add the "indexer" worker yet, since it's nontrivial and needs some careful design.

[msm-cert]

On the other hand, this idea needs some design. It's possible that the feature is too powerful and I should just special-case indexing s3 and local disk (because that's what everyone uses).

Finishing #432 correctly will probably require ursadb changes.