Add checkds code

Add checkds code

Similar to notify, add code to send and keep track of checkds requests.

On every zone_rekey event, we will check the DS at parental agents
(but we will only actually query parental agents if theree is a DS
scheduled to be published/withdrawn).

On a zone_rekey event, we will first clear the ongoing checkds requests.
Reset the counter, to avoid continuing KSK rollover premature.

This has the risk that if zone_rekey events happen too soon after each
other, there are redundant DS queries to the parental agents. But
if TTLs and the configured durations in the dnssec-policy are sane (as
in not ridiculous short) the chance of this happening is low.