Logic to support optionally keeping or removing the CDS/CDNSKEY RRs on the signers.

[romu42]

Currently the CDS/CDNSKEY RRs are removed from the signers in the signergroup once the DS records at the parent are synced and verified.

• Decide if we should keep this logic, this entails a prerequisite that the signers do not inject CDS/CDSNSKEY RRs themselves and that MUSIC is the only entity handling these records.
• If we want to allow the signers to handle the CDS/CDNSKEY RRs as well we need logic in MUSIC to deal with thisl.