validation: Move LoadBlockIndexDB to CChainState

Summary:
CChainState needed cuz setBlockIndexCandidates

This is a backport of core#21055 [9/16]
https://github.com/bitcoin/bitcoin/pull/21055/commits/8cdb2f7e58dfd9a631a8cbb8f0ee7e8c0c304eb4#

Depends on D11213

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11214