Require IsBlockPruned() to hold mutex cs_main

Summary:
Co-authored-by: Vasil Dimov <vd@FreeBSD.org>

This is a partial backport of core#22932
https://github.com/bitcoin/bitcoin/pull/22932/commits/eaeeb88768db529b5241ccd42f1e87579908b4df

Depends on D13036

Test Plan:
With clang and DEBUG:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13037