Move LoadBlockIndexDB to BlockManager

Summary:

The block tree db is used within BlockManager to write and read the block index, so make the it a member variable of BlockManager instead of a global.

This is a backport of core#22371 [1/2]
https://github.com/bitcoin/bitcoin/pull/22371/commits/fa27f03b4943540aa2eab283d4cf50ad4a1a01f8

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12241