CCoinsCacheEntry provided general access to its internal flags state, even though in reality it could only be clean, fresh, dirty or fresh|dirty.
After it got dirtied we couldn't set the state back to clean by AddFlags(0) - tests were explicitly checking against that.
This commit cleans up the public interface to make this distinction cleaner and invalid behavior impossible instead of just checked by tests.
This includes the removal of redundant `inline` qualifiers (we're inside a struct).
Also renamed `self` to `pair` to simplify the upcoming commits.
Also modernized `EmplaceCoinInternalDANGER` since it was already modified.
Co-authored-by: Andrew Toth <andrewstoth@gmail.com>
This is a partial backport of core#30906
https://github.com/bitcoin/bitcoin/pull/30906/commits/cd0498eabc910efa3ed7a6d32e687107248bb5be