coins, refactor: Make AddFlags, SetDirty, SetFresh static

Summary:
This makes the Assume(&self.second == this) check redundant

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

This is a partial backport of core#30906
https://github.com/bitcoin/bitcoin/pull/30906/commits/fc8c282022e6ce4eb3ce526800a6ada3b4a2996d

Depends on D18806

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18807