Use virtualsize for mining/mempool priority

Summary:
This uses the virtualsize for influencing the mining priority / mempool eviction
priority under conditions of mempool congestion.

All other uses of size in mining/mempool continue to use actual size:

• mempool entry minfeerate limitation (the 1 sat/byte regular limit)
• mining minimum feerate calculation
• mining block size calculations
• ancestor size limit for chains
• descendant size limit for chains
• sorting order for net_processing mempool requests
• calculation of 'bytes' in getmempoolinfo

This changes no behaviour under normal conditions, only under congestion.

Depends on D4906

Test Plan: ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4903