mempool: Evict transactions by lowest modified fee rather than feerate-with-descendants

This changes transaction eviction policy to remove transactions by lowest modified fee, rather than feerate-with-descendants. The descendant_score index is removed, as its only use was in TrimToSize. Instead, we can re-use the index added in !1079 (merged).

Pulled out of @jtoomim's cpfp-nuke branch.

Benchmark comparison of TrimToSize before and after, using the EvictChained* benchmarks which use a filled mempool

Before: (master @ed768039))

EvictChained50Tx, 5, 2, 21.0273, 2.1008, 2.10495, 2.1029
EvictChained50TxRev, 5, 2, 16.1526, 1.6127, 1.61777, 1.61571

After: (!1080 (merged) rebased on master @ed768039)

EvictChained50Tx, 5, 2, 15.5157, 1.54984, 1.55343, 1.5516
EvictChained50TxRev, 5, 2, 14.8455, 1.48256, 1.48721, 1.48475

Test plan

ninja check-bitcoin check-functional

Merge strategy: Do NOT squash.