Add code to store SKR

Add code to store SKR

This added source code stores SKR data. It is loosely based on:
https://www.iana.org/dnssec/archive/files/draft-icann-dnssec-keymgmt-01.txt

A SKR contains a list of signed DNSKEY RRsets. Each change in data
should be stored in a separate bundle. So if the RRSIG is refreshed that
means it is stored in the next bundle. Likewise, if there is a new ZSK
pre-published, it is in the next bundle.

In addition (not mentioned in the draft), each bundle may contain
signed CDS and CDNSKEY RRsets.

Each bundle has an inception time. These will determine when we need
to re-sign or re-key the zone.