cert: speed up cert-find
    
    Use issuer+serial rather than raw DER blob to identify certificates in
    cert-find's intermediate result.
    
    Restructure the code to make it (hopefully) easier to follow.
    
    https://fedorahosted.org/freeipa/ticket/6098
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>