getmempool mempoolminfee is a BCH/kB feerate
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

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