Move ::fCheckBlockIndex into ChainstateManager

Summary:
This changes the flag for the bitcoin-chainstate executable. Previously
it was false, now it is the chain's default value (still false for the
main chain).

This concludes backport of core#25704
https://github.com/bitcoin/bitcoin/pull/25704/commits/fa7c834b9f988fa7f2ace2d67b1628211b7819df

Depends on D15814

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15815