Store ForestCAR.zst files directly in the database

[lemmih]

Issue summary

When importing a CAR, CAR.zst, or ForestCAR.zst file, we should convert it to ForestCAR.zst and place it directly in the database folder. ForestCAR.zst files are smaller and faster than ParityDB. They're read-only and dead information cannot be garbage collected, but that matters little compared to the benefits of using compressed data.

Acceptance criteria:

• Convert imported CAR and CAR.zst files into ForestCAR.zst. ForestCAR.zst files obviously do not have to be re-converted. Move the file to the database folder.
• Opening a database should give a block store that is the union of ParityDB and all available ForestCAR.zst files.
• Add flag --consume-snapshot. It should do the same as --import-snapshot, but it'll move or delete the snapshot.

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