[feature] Offline generation of Parity Warp Sync Snapshots

[riptl]

Request: Add a geth warp subcommand that generates a Parity-compatible "v2" snapshot of the last N blocks and a single state trie, while the node is offline.

The Warp Sync Snapshots used by OpenEthereum are fairly simple to generate, ignoring parallelization for now.
https://openethereum.github.io/wiki/Warp-Sync-Snapshot-Format

The major implementation steps:

1. Collect blocks with uncles and receipts and PoW metadata.
2. Iterate the accounts trie, save accounts, code and the storage trie.
3. A set of writers that do chunking, compression and RLP streaming.
4. Port OpenEthereum tests.

This issue does not cover

• building the snapshots during node operation (overlap with Dynamic State Snapshots)
• "seeding" the snapshots via P2P
• restoration of warp snapshots

I'm fairly sure that no changes to the other packages (eth/core, trie) are required. The only external dependency, snappy compression, is already pulled in.

I'd like to work out my proof of concept into a PR if this feature has any chance getting merged.
If not (maybe GPLv3 vs LGPLv3 licensing issues?), I'm going to keep it out-of-tree instead.

[karalabe]

Geth already ships (though not yet enabled by default) a superset of the Parity snapshot (#20152) that can also be used for EVM acceleration and Merkle proven state syncs (ethereum/devp2p#145) .It also seems to perform crazy well (https://twitter.com/peter_szilagyi/status/1264801013174845442).

All in all I don't think there's any value for us to support Parity's "legacy" snapshot format. It would just be code we'd need to maintain for no benefit.

[MariusVanDerWijden]

@karalabe I really like parity's documentation about the snapshot format. Maybe we can add a page to our documentation to document our format.