coins: track flagged cache entries in linked list

Summary:
No visible behavior change. This commit tracks the flagged
entries internally but the list is not iterated by anything.

Co-Authored-By: Pieter Wuille <pieter@wuille.net>
Co-Authored-By: l0rinc <pap.lorinc@gmail.com>

This is a partial backport of core#28280
https://github.com/bitcoin/bitcoin/pull/28280/commits/24ce37cb867b95e86d9fd4e50858d64ee8a59abf
https://github.com/bitcoin/bitcoin/pull/28280/commits/a14edada8a051e280af6fedd5130be40247e2d7a
Depends on D18616

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18617