Compute the adds and deletes required to get from existing RRData to new RRData
and perform just those operations instead of doing a complete drop/add of the
entire RRData set. Also during an update, do adds before deletes. This
eliminates the existence gap and fixes#29
Abstract out the nsupdate provider logic and implement a new resource_record
type around it which applies no semantic value to its title.
Also: support SPF and TXT records, include isrequired on required parameters
(though it doesn't seem to work as advertised)
