MOVEONLY: mempool checks to their own functions

Summary:

No change in behavior

This is a partial backport of core#23381
https://github.com/bitcoin/bitcoin/pull/23381/commits/c9b1439ca9ab691f4672d2cbf33d9381f2985466

Depends in D12359

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12360