This makes CheckInputs do the following:
- If metrics are found in cache, return them.
- If cache miss and checks are not deferred, then accumulate the total of inputs' metrics, store them in cache if requested, and then finally return those.
- If cache miss and checks are deferred, return empty metrics.
Due to D4940 even though some standard txes right now may have huge sigchecks
counts > 65535, they will still be cacheable so there is no performance penalty.
Depends on D4940 and parents