This will allow us to pass e.g. a ResourceRecordChangeset, rather than a
ResourceRecordChangeset, the parent ResourceRecordSets, and the
grandparent Zone.
Laying the groundwork for simplifying / optimizing the federation logic.
Although Google Cloud DNS requires strict add & remove calls, most
dnsproviders actually support upsert, and an add & remove is much more
expensive (primarily because of the need to fetch the pre-image).
Add support for 'upsert' operations, which don't require the pre-image,
and simply overwrite the existing record. This is much cheaper on
Amazon Route53, for example.
Automatic merge from submit-queue
dnsprovider: Add IsEmpty method
When batching changes, it is often handy to know whether a changeset
IsEmpty, and thus does not need to be Apply-ed.
```release-note
NONE
```
