mempool: Add new index for sorting txs by fee
Summary:
This adds a new mempool index that sorts transactions by modified fee rate, ignoring ancestors.
Port of bchn#1079.
The comparator has been modified so that 2 different txs with the same
modified feerate and time will NOT be considered equal. This makes the
comparator usable for use in sets or maps.
Depends on D13109.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D13110