> refactor: use const reference for parents in `CTxMemPool::UpdateAncestorsOf`
> refactor: move update_* structs from txmempool.h to .cpp file
>
> These helpers are exclusively used in txmempool.cpp, hence they
> should also be moved there.
>
> Can be reviewed with "--color-moved=dimmed-zebra".
This is a backport of core#23211