[FEAT] - design a lightweight checkpointing mechanism

[nfrisby]

Even once Ouroboros Genesis is released, "disasters" or "inexplicably lucky" attackers may give rise to scenarios where a lightweight checkpointing mechanism would be useful. Notably, the mere existence of such a mechanism might even itself make the attack vector less attractive.

This Issue is to design such a mechanism.

At this point, the rough idea is that some configuration file could contain a list of points (ie pairs of a slot and a hash) that overrides normal chain comparison and/or validation in some way. The fundamental candidates are: EITHER a header/block whose point is in the list is considered invalid by fiat OR all headers/blocks in the same slot as one of the points but with a different hash are considered invalid by fiat.

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This is explicitly not a heavyweight checkpointing mechanism, eg once that would give rise to a new and distinct genesis block.

[nfrisby]

So far I've received feedback wondering if Mithril supplants this. I look forward to learning more about Mithril during the Consensus Office hours next week (2023 Oct 12), after which I can hopefully compare and contrast them here.

Edit: my initial guess is that Mithril would supplant it, but that creating Mithril snapshots is more involved than announcing a list of points. However, even "announcing" a list of points would require a process for signing them, so a significant portion of the overall Mithril process' "weight" is probably necessary for the lightweight mechanism of this Issue to actually be utilized by IOG/EMURGO/CF/etc.

[amesgen]

Duplicate of #449