Limit the number of sigcheck added to a block when mining

Summary: Previously, we used the consensus limit, but if a miner selected a smaller limit it is only consistent to limit the sigcheck as well accordingly.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8554