move-only: Create WriteBlockIndexDB helper

Summary:
Can be reviewed with --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space

This is a partial backport of core#23974
https://github.com/bitcoin/bitcoin/pull/23974/commits/fa467f3913918701c765f9bc754203b4591b894f

Depends on D12510

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12511