This avoids needing to send the entire certificate in OCSP generation
RPCs.
Ended up including a few cleanups that made the implementation easier.
Initially I was struggling with how to derive the issuer identification info.
We could just stick the full SPKI hash in certificateStatus, but that takes a
significant amount of space, we could configure unique issuer IDs in the CA
config, but that would require being very careful about keeping the IDs
constant, and never reusing an ID, or we could store issuers in a table in the
database and use that as a lookup table, but that requires figuring out how to
get that info into the table etc. Instead I've just gone with what I found to
be the easiest solution, deriving a stable ID from the cert hash. This means we
don't need to remember to configure anything special and the CA config stays
the same as it is now.
Fixes#4469.
b557d870c7