calculate data rather than using the revoked flag

Cascade delete of CdnCertificate to the Serial
This way we never end up putting CdnCert serials in the crl.
Probably doesn't make much difference, but this seems cleaner
index cdn certificates on serial_id for uniformity with other certs
Merge pull request #582 from candlepin/ckozak/rework_revoked

calculate data rather than using the revoked flag