Require CBlockIndex::IsValid() to hold cs_main

Summary:
This is a partial backport of core#22932
https://github.com/bitcoin/bitcoin/pull/22932/commits/ca47b005770f71aa229ecc1f7b8146a96ff02151

Depends on D13035

Test Plan:
With clang and DEBUG:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13036