add sigchecks limiter to CheckInputs

Summary:
Unused so far, this will be used to implement the block
level limit on sigchecks.

(A second one could be added the same way later to limit both per-
transaction and per-block at the same time, not worrying about that
for now.)

Test Plan: ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Maniphest Tasks: T704

Differential Revision: https://reviews.bitcoinabc.org/D5127