edge

Fix zone detection for cross-zone cnames (#449)

* Fix zone detection for cross-zone cnames

CNAMEs cannot co-exist with SOA records so responses with
a CNAME should be skipped.

The `cross-zone-example.assets.sh.` is currently hosted by
me (@fd) and will continue to exist for as long as the assets.sh
domain exists. (The assets.sh domain is used as a CDN and is unlikely
to go away.)

See #330

* Extracted CNAME checking to simplify the FindZoneByFqdn control flow.