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 D4897 xor D4906