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