refactor: Remove confusing BlockIndex global

Summary:

The global ::BlockIndex() is problematic for several reasons:

• It returns a mutable reference to the block tree, without the appropriate lock annotation (m_block_index is guarded by cs_main). The current code is fine, but in the future this might lead to accidental races and data corruption.
• The rpc server shouldn't rely on node globals, but rather a context that is passed in to the RPC method.
• Tests might want to spin up their own block tree, and thus should also not rely on a single global.

Fix all issues by removing the global

This is a backport of core#19413

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9581