index: Ensure block locator is not stale after chain reorg.

Summary:
This is partial backport of Core PR14121 : https://github.com/bitcoin/bitcoin/pull/14121/commits/62b7a4f09409f60a35fc99ddb70f9199ac511a53

Depends on D6332

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6333