add a flag that restricts sigChecks per-input

Summary:
This flag is intended for standardness-only, and the primary goal is to
make is so that malicious super-high-density-SigChecks transactions cannot
get into the mempool, where they would frustrate simple block assembly
algorithms.

A transaction conforming to this rule cannot (overall) exceed a density of
1 sigcheck per 36.666 bytes.

(ref: https://gist.github.com/markblundeberg/becf85ac6ba48bebfeb7871ce2de27a7)

Depends on D4835

Test Plan:

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T704

Differential Revision: https://reviews.bitcoinabc.org/D4617