Introduce MemPoolAccept::CheckFeeRate

Summary: This was skipped during the backport of core#16400 because it was not really needed. I'm adding it now because it makes the later backports easier and it doesn't change the behavior.

Test Plan:

ninja all check-extended

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16370