Move pblocktree global to BlockManager
Summary:
This is a backport of core#22371 [2/2]
https://github.com/bitcoin/bitcoin/pull/22371/commits/faa54e375782b21cbc2761c763128131c569e903
Depends on D12241
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D12242