Add a zsk/ksk rollover detector

I think that should be a separate goroutine that periodically scans the DNSKEY RRsets for all zones under management.

The scan interval should be configured via the config file.

If a new key is detected some sort of update should be sent to the db updater.

If it is a zsk rollover then my current thinking is that zsk rollovers are completely orthogonal to the other processes. I.e. the zone should run a zsk rollover process (which is short and simple) in parallel with whatever other process that may or may not be going on at the time.

If it is a ksk rollover then things get more complicated.

If the zone is not running any process then we're good (run the ksk-rollover process independently).
If we're in an "add-signer" process then I think we need to jump back to just before"CDS-KNOWN" to re-compute the CDS and then progress from there.
If we're in an "REMOVE-SIGNER" process and we have already passed the "CDS-KNOWN" state then we jump back. If we're not yet at the CDS-KNOWN state we do nothing, as we will pick up on the new KSK when we get there.