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