scripted-diff: Rename SetBestChain callback ChainStateFlushed

Summary:
backport of first commit (50b6533) from PR13106

Note from Mark -- Matt missed renaming in a couple of comments--fixed => backport of core PR13154.

Depends on D2904

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, Fabien, Mengerian

Reviewed By: #bitcoin_abc, Fabien

Maniphest Tasks: T632

Differential Revision: https://reviews.bitcoinabc.org/D2927