[refactor] put finality and maturity checking into a lambda

Summary:
No behavior change.

This is a partial backport of core#22677
https://github.com/bitcoin/bitcoin/pull/22677/commits/bb9078ed51159fa162484f16993313ed6cf980e3

Depends on D12441

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12443