refactor: encapsulate CCoinsCacheEntry flags access

Summary:

refactor: encapsulate flags access for dirty and fresh checks

No behavior change. This prepares moving the cache entry flags field
to private access.

Co-Authored-By: l0rinc <pap.lorinc@gmail.com>

https://github.com/bitcoin/bitcoin/pull/28280/commits/df34a94e57659c31873c26c86a8de5a032400061

refactor: encapsulate flags get access for all other checks

No behavior change. This prepares moving the cache entry
flags field to private access.

https://github.com/bitcoin/bitcoin/pull/28280/commits/9715d3bf1e4013476539f1523a6b54d2055c32c6

refactor: encapsulate flags setting with AddFlags and ClearFlags

No behavior change. This prepares moving the cache entry
flags field to private access.

https://github.com/bitcoin/bitcoin/pull/28280/commits/8737c0cefa6ec49a4d17d9bef9e5e1a7990af1ac

This is a partial backport of core#28280

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18604