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