[backport#16194] refactoring: add block_index_candidates arg to LoadBlockIndex

Summary:
Prevents BlockManager from having to reference ChainstateActive()
within one of its methods which improves encapsulation and makes
testing easier.

https://github.com/bitcoin/bitcoin/pull/16194/commits/4ed55dfcd7894fd5ba6395f244a17ab1f8e786d4

Depends on D6959

Partial backport of Core PR16194

Test Plan:

ninja check && ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6967