Move ::hashAssumeValid into ChainstateManager

Summary:
This changes the assumed valid block for the bitcoin-chainstate
executable. Previously it was BlockHash{}, now it is defaultAssumeValid.

This is a partial backport of core#25704 and core#26409
https://github.com/bitcoin/bitcoin/pull/25704/commits/fa29d0b57cdeb91c8798d5c90ba9cc18085e99fb
https://github.com/bitcoin/bitcoin/pull/26409/commits/fa29ef00adac6f0842acdd38344820a1ce0e3087

Depends on D15811

Test Plan:
ninja all check-all

Run IBD

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15812