Merge #12980: Allow quicker shutdowns during LoadBlockIndex()

Summary:
af5fa82b6 Allow quicker shutdowns during LoadBlockIndex() (Jonas Schnelli)

Pull request description:

ACKs for commit af5fa8:

promag:
  utACK af5fa82b676a36e60eda080ca0a946bdfffefd49.
practicalswift:
  utACK af5fa82b676a36e60eda080ca0a946bdfffefd49

Tree-SHA512: 1c64dcc5d8a9d3411553257cd5a598dcd29be981660e5bca9283c1d957dc56798abcf41d9969cd573088137597a23e48e62a8c476c463d3f176b86a10048f47b

Backport of Core PR12980

Test Plan: ninja check check-functional for sanity

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6513