Make blockstorage globals private members of BlockManager

Summary:
This is a partial backport of core#23974
https://github.com/bitcoin/bitcoin/pull/23974/commits/facd3df21f344dd84e5f28862056700c1fded17c

Depends on D12514

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12517