refactor: simplify ChainIndex, hoist out checkpoints

[lemmih]

Summary of changes

Changes introduced in this pull request:

• Get rid of the skip-link code. It was used to query really old epochs but we don't need it.
• Move tipset_by_height from ChainStore to ChainIndex.
• Refactor the known blocks (aka checkpoints) to be simpler. They no longer need to be verified in the daemon (can be handled offline.)
• Add new forest-cli archive checkpoints command for generating new checkpoints.

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[lemmih]

This needs to be reviewed first: #3216

[aatifsyed]

Generally happy with these changes, only serious comment is #3220 (comment)

[aatifsyed]

new issue:

I'd like to remove the serialization for this, and turn it into rust code, probably using something like databake, but that's a little heavyweight - we could then just

struct KnownBlocks<'a> {
    calibnet: &'a [(i64, Cid)],
    mainnet: &'a [(i64, Cid)],
}
const KNOWN_BLOCKS: KnownBlocks<'static> = todo!();

Unfortunately, multihash and cid aren't there yet:
multiformats/rust-cid#138
multiformats/rust-multihash#330

[aatifsyed]

#3227

[LesnyRumcajs]

Why should it be Rust code?

[aatifsyed]

It's less work, and makes an error state impossible.
Consider this:

use std::net::Ipv4Addr;

const MEH: &str = "127.0.0.1";
const NICE: Ipv4Addr = Ipv4Addr::new(127, 0, 0, 1);

fn meh() -> Ipv4Addr {
    MEH.parse().unwrap()
}

fn nice() -> Ipv4Addr {
    NICE
}

Why should meh() need to unwrap, when it can be NICE?
NICE also facilitates inlining, and all that other good stuff :)

[aatifsyed]

Even the simple case can't be const-evaluated by the compiler: https://godbolt.org/z/ooqGrhdxE

[LesnyRumcajs]

rock-solid