refactor: Move load block thread into ChainstateManager

Summary:
This is a backport of core#21575 [1/6]
https://github.com/bitcoin/bitcoin/pull/21575/commits/faf843c07f99f91603e08ea858f972516f1d669a

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11347