deprecate rrset.rdata in favor of the more intuitive rrset.records

Particularly as the next version of denominator will have json and yaml representations, we need to ensure our field names are as intuitive as possible.

`ResourceRecordSet.rdata()` currently returns a list of, for example ip addresses.
`ResourceRecordSet.records()` is more intuitive at the cost of being slightly less "spec correct" 

Thanks @kiall for the idea.