feat: chain import: don't walk to genesis - 2-3x faster snapshot import

[magik6k]

Related Issues

This cherrypicks 921a3a0 from #10821

Proposed Changes

• Snapshot cars are written from chain top to bottom
• We know what the top block header is because that's the car root
• We know that the links in that block header will lead us to the correct parent
• So when we see parents of the last seen block, we record those
• Snapshots contain all block headers to genesis, so if we repeat this process for the whole import we never have to read a single block to find the genesis block
• This way we can instantly set the correct genesis after snapshot import without having to walk there, saving roughly 17 000 000 useless reads from chainstore
  • (This is what causes the hand after chain import after the 'flushing validation cache' log)
  • For me it makes the mainnet snapshot import 2-3x faster

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• If the PR affects users (e.g., new feature, bug fix, system requirements change), update the CHANGELOG.md and add details to the UNRELEASED section.
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green

[Stebalien]

What about the json genesis thing? (I assume you've tested this, I just wanted to make sure we're not missing something)

[magik6k]

The first block with the BlockHeader type is at height 0, so here we just check for that - https://filfox.info/en/block/bafy2bzacecnamqgqmifpluoeldx7zzglxcljo6oja4vrmtj7432rphldpdmm2