getmempool mempoolminfee is a BCH/kB feerate
Summary:
Backport of core PR10475
https://github.com/bitcoin/bitcoin/pull/10475/files
Test Plan:
bitcoin-cli help getmempoolinfo
Reviewers: #bitcoin_abc, deadalnix, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D3430