validation: Make LoadBlockIndex() a member of ChainstateManager

Summary:
Partial backport (3/6) of core PR18698:
https://github.com/bitcoin/bitcoin/pull/18698/commits/fa84b1cd846f6499b741710fd478ec9ad49b5120

Depends on D8561.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8562