IPv6 fix

[yoannchaudet]

This PR fixes logic related to our recent rollout of IPv6 for Pages.

AAAA record presence is now also looking at the actual record name not to be confused by a CNAME resolution.

For instance in:

dig AAAA pages.github.com

; <<>> DiG 9.10.6 <<>> AAAA pages.github.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35585
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;pages.github.com.		IN	AAAA

;; ANSWER SECTION:
pages.github.com.	3600	IN	CNAME	github.github.io.
github.github.io.	1451	IN	AAAA	2606:50c0:8000::153
github.github.io.	1451	IN	AAAA	2606:50c0:8001::153
github.github.io.	1451	IN	AAAA	2606:50c0:8002::153
github.github.io.	1451	IN	AAAA	2606:50c0:8003::153

We used to consider AAAA record were present on the zone while they are not. They are resolved via the CNAME record.

cc/ https://github.com/github/pages-engineering/issues/787