Add a new `GetRevocationStatus` gRPC method to the SA which retrieves
only the subset of the certificate status metadata relevant to
revocation, namely whether the certificate has been revoked, when it was
revoked, and the revocation reason. Notably, this method is our first
use of the `goog.protobuf.Timestamp` type in a message, which is more
ergonomic and less prone to errors than using unix nanoseconds.
Use this new method in ocsp-responder's checked_redis_source, to avoid
having to send many other pieces of metadata and the full ocsp response
bytes over the network. It provides all the information necessary to
determine if the response from Redis is up-to-date.
Within the checked_redis_source, use this new method in two different
ways: if only a database connection is configured (as is the case today)
then get this information directly from the db; if a gRPC connection to
the SA is available then prefer that instead. This may make requests
slower, but will allow us to remove database access from the hosts which
run the ocsp-responder today, simplifying our network.
The new behavior consists of two pieces, each locked behind a config
gate:
- Performing the smaller database query is only enabled if the
ocsp-responder has the `ROCSPStage3` feature flag enabled.
- Talking to the SA rather than the database directly is only enabled if
the ocsp-responder has an `saService` gRPC stanza in its config.
Fixes#6274
09195e6804