Since 9906c93217 when
`features.PrecertificateOCSP` is enabled it is possible for there to be
`certificateStatus` rows that correspond to `precertificates` that do not have
a matching final `certificates` row. This happens in the case where we began
serving OCSP for a precert and weren't able to issue a final certificate.
Prior to the fix in this branch when the `ocsp-updater` would find stale OCSP
responses by querying the `certificateStatus` table it would error in
`generateResponse` when it couldn't find a matching `certificates` row. This
branch updates the logic so that when `features.PrecertificateOCSP` is enabled
it will also try finding the ocsp update DER from the `precertificates` table
when there is no matching serial in the `certificates` table.