move-mostly: Make fHavePruned a BlockMan member

Summary:
[META] In the next commit, we move the clearing of fHavePruned to BlockManager::Unload()

This is a partial backport of core#24909
https://github.com/bitcoin/bitcoin/pull/24909/commits/3308ecd3fc254ee4ef9f803c09f00ba4dc968520

Depends on D13045

Test Plan: ninja all check-all bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13046