refactor: move update_* structs from txmempool.h to .cpp file

Summary:

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

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12287