policy / validation: CPFP fee bumping within packages
Summary:
This enables CPFP fee bumping in child-with-unconfirmed-parents packages by introducing package feerate (total modified fees divided by total virtual size) and using it in place of individual feerate. We also always validate individual transactions first to avoid incentive-incompatible policies like "parents pay for children" or "siblings pay for siblings" behavior.
Backport of core#24152.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, roqqit
Reviewed By: roqqit
Subscribers: roqqit
Differential Revision: https://reviews.bitcoinabc.org/D16378