core: speed up header import (#21967)

This PR implements the following modifications

- Don't shortcut check if block is present, thus avoid disk lookup
- Don't check hash ancestry in early-check (it's still done in parallel checker)
- Don't check time.Now for every single header

Charts and background info can be found here: https://github.com/holiman/headerimport/blob/main/README.md
With these changes, writing 1M headers goes down to from 80s to 62s.