Require CBlockIndex::IsAssumedValid() to hold cs_main

Summary:
This is a partial backport of core#22932
https://github.com/bitcoin/bitcoin/pull/22932/commits/8ef457cb83fac796f8b6a56977b1016193fc1185

Depends on D13034

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/D13035