validation: Use existing chain member in CChainState::AcceptBlock and LoadGenesisBlock

Summary:
This is a backport of core#21055 [14 & 15/16]
https://github.com/bitcoin/bitcoin/pull/21055/commits/5e4af773809415b71a10e3120cc44854d61c4c19
https://github.com/bitcoin/bitcoin/pull/21055/commits/03f75c42e12a272057adccb6f0077e71f971eeef

Depends on D11220

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11221