move-mostly: Make pindexBestHeader a ChainMan member

Summary:
[META] In the next commit, we move the clearing of pindexBestHeader to ChainstateManager::Unload()

This is a partial backport of core#24909
https://github.com/bitcoin/bitcoin/pull/24909/commits/0d567daf23c9fcb2d95b38913ee45a8b0ba3b027

Depends on D13043

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13044