track descendant sigops count in mempool
Summary:
This is needed if it is desired to do accurate virtualsize accounting of
packages (virtualsize based on resource totals computed separately, not
just sum of virtualsize). See D4906.
Depends on D4904
Test Plan: ninja check
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4905