refactor: Separate hash and stats calculation in coinstats

Summary:
This is a backport of core#19145 [2/3]
https://github.com/bitcoin/bitcoin/pull/19145/commits/2474645f3b15687e7f196b89eb935d6e6a98a9da

Depends on D11155

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11156