validation: Make CChainState.m_blockman public

Summary:
This is a backport of core#20749 [9/17]
https://github.com/bitcoin/bitcoin/pull/20749/commits/0e17c833cda67cdba5338bd7409061772b6d5edb

Depends on D11177

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11180