track nSigChecks in CheckInputs

Summary:
This makes CheckInputs do the following:

• If sigchecks are found in cache, return them.
• If cache miss and checks are not deferred, then accumulate the total of inputs' sigchecks, store them in cache if requested, and then finally return those.
• If cache miss and checks are deferred, return 0 sigchecks.

Depends on D4940 and D4834

Test Plan: ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T704

Differential Revision: https://reviews.bitcoinabc.org/D5126