Move pindexBestInvalid and pindexBestParked to ChainstateManager

Summary:
A private member is better than a global.

This concludes backport of core#23785
https://github.com/bitcoin/bitcoin/pull/23785/commits/fab6d6b2d154893ab422dda87f3535d42c3e06f4

Depends on D12486

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12487