Archive completed certificate requests

[bbengfort]

At the moment, completed certificate requests are marked as completed but must be kept so that the details of the batch and the pkcs12 password might be used to recover issued certificates. However, this means that the CertManager go routine will loop over a growing number of certificate requests over time. We need to decide if keeping the certificate requests for recovery is a valid use case and:

• if not: destroy certificate requests when they are completed
• if so: archive certificate requests to a different prefix so that they are not scanned by the CertManager