Swap relay pool and mempool lookup

Summary:
This is in preparation to using the mempool entering time as part of
the decision for relay, but does not change behavior on itself.

This is a partial backport of core#19109 [1/5]
https://github.com/bitcoin/bitcoin/pull/19109/commits/a9bc5638031a29abaa40284273a3507b345c31e9

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9434