Control default TTL

[codefromthecrypt]

ResourceRecordSet.ttl can be null to suggest inherit a default. This could use provider-specific functionality or instead lookup the SOAData.ttl and apply it.

This was attempted in #351, but not practical due to lack of support.

• route53 - rrsets require an explicit ttl param, so denominator hard-codes
• ultradns - default TTL per record type in the SOAP API, see: saveAccountPreferences. regardless, we currently hard-code
• google cloud dns - rrsets require an explicit ttl param
• dynect - default is SOA.ttl
• clouddns - default is zone.ttl
• designate - default is zone.ttl

Using the SOAData.ttl in lieu of provider support would make this feature expensive to implement, as that would imply we need to lookup or cache the SOA for any RRSet update that set the ttl to null. We could also make a denominator-specific default obtained from configuration.

[codefromthecrypt]

Cleared out comments after reworking title and description