Update mempool fee estimator to factor in rollingMinimumFeerate

Summary:
minerPolicyEstimator uses recent blocks to figure out a reasonable fee. This may disagree
with the rollingMinimumFeerate under certain scenarios where the blocks include low fee
transactions, but the mempool is being spammed.

This commit updates the mempool fee estimator to use the maximum of the two values.

Test Plan:

make check && ./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, deadalnix, schancel

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: jasonbcox, deadalnix, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2037