mempool: Evict transactions by modified feerate

Summary:

When trimming the mempool, remove transactions by lowest (modified) feerate rather than feerate-with-descendants.

Port of bchn#1080.

Depends on D13116 and D13120.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK, sdulfari

Reviewed By: #bitcoin_abc, PiRK, sdulfari

Subscribers: sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13121