Completely refactor the way we organize our code related to OCSP.
- Move it all into one `//ocsp/` package, rather than having multiple
top-level packages.
- Merge the OCSP updater's config sub-package with its parent
(since it isn't necessary to break it out to avoid cyclic imports).
- Remove all `Source` logic from ocsp-responder's `main.go`, because
it was difficult to mentally trace the control flow there.
- Replace that logic with a set of composable `Source`s in the
`//ocsp/responder/` package, each of which is good at just one thing.
- Update the way the filters work to make sure that the request's
`IssuerKeyHash` and the response's `ResponderName` can both
be derived from the same issuer certificate, ensuring that the req and
resp are correctly matched.
- Split the metrics into a separate metric for each `Source`, so we can
tell what all of them are doing, not just aggregate behavior.
- Split the tests into individual files for each `Source`, and update them
for the new public interfaces.
Aaron Gable