validation: Move global LookupBlockIndex method to BlockManager
Summary:
LookupBlockIndex only acts on BlockManager.
This is a backport of core#20749 [2, 3 & 4/17]
https://github.com/bitcoin/bitcoin/pull/20749/commits/15d20f40e1321b24963b40c12958c7d30ad112ec
https://github.com/bitcoin/bitcoin/pull/20749/commits/eae54e6e60d7ed05b29d8345c0bb055397149ce8
https://github.com/bitcoin/bitcoin/pull/20749/commits/3664a150ac7547c9336b571557af223d9e31aac9
Depends on D11172
Test Plan:
With debug:
ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D11173