Faster and simpler inflation check with coinstatsindex

[Sjors]

A proposed feature, perhaps available in Bitcoin Core v0.21, adds an index for block stats, which makes gettxoutsetinfo blazing fast. It should also allow calling it for a given block.

bitcoin/bitcoin#18000

This allows for bypassing the complicated rollback mechanics on our mirror nodes: https://github.com/BitMEXResearch/forkmonitor/blob/master/app/models/inflated_block.rb#L64

[Sjors]

bitcoin/bitcoin#19521 is making good progress.

Once merged, I should be able to rebase our (taproot wallet enabled) custom fork of Bitcoin Core, so we can start using this.

The mirror nodes and the rollback mechanism is still useful for checking validity of stale blocks.

[Sjors]

The coinstats index has been in Bitcoin Core for a while now. It's already turned on for our main 24.0.1 node. I just haven't had a chance to implement this.