As part of that, add support for issuer IDs in orphan-finder's
and RA's calls to GenerateOCSP.
This factors out the idForIssuer logic from ca/ca.go into a new
issuercerts package.
orphan-finder refactors:
Add a list of issuers in config.
Create an orphanFinder struct to hold relevant fields, including the
newly added issuers field.
Factor out a storeDER function to reduce duplication between the
parse-der and parse-ca-log cases.
Use test certificates generated specifically for orphan-finder tests.
This was necessary because the issuers of these test certificates have
to be configured for the orphan finder.
Jacob Hoffman-Andrews