Implement fallback blockstore

[hanabi1224]

Summary

Lotus has a fallback blockstore that can be enabled by setting LOTUS_ENABLE_CHAINSTORE_FALLBACK=1, we need to implement the same in Forest.

The fallback blockstore uses bitswap to fetch missing blocks from the p2p network

Completion Criteria

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Additional Links & Resources

[LesnyRumcajs]

Do you think there are some security considerations we should have in place for this feature? E.g., fetching missing blocks for random tipset that a malicious node published could potentially exhaust the node's memory/disk space, right?

[hanabi1224]

@LesnyRumcajs I think that's a valid concern. We could follow Lotus to disable this feature by default. This is useful for powering Curio tho.

[0xDevNinja]

Hi, I'd like to work on this issue. Could you please assign it to me? Thanks!