Improve the mempool check method

Summary:
This makes the mempool check more readable, especially by using better names than it1, it2, it3 etc. and by not reusing the names through the method.
There is no change in behavior.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13440