Libbitcoin for Core people

[evoskuil]

@andrewtoth

Looking at a flamegraph of Bitcoin Core IBD, the majority of time is spent looking up input prevouts. That’s why parallelizing that provides a nice speedup. Skipping it entirely though is not possible since we would not have a utxo set at the end, which is not a constraint that libbitcoin has.

I’m sure it could be a material improvement.