feat: explicitly query fallback servers for NS records

[agaffney]

Fixes #29

[agaffney]

$ LOGGING_LEVEL=debug ./cdnsd 
{"level":"info","timestamp":"2023-10-04T19:56:28-05:00","caller":"indexer/indexer.go:64","msg":"found previous chainsync cursor: 34352991, 06bf848293c5fedb326134351ea0b68efae0445c1332929665ecd25b46d0c92b"}
{"level":"info","timestamp":"2023-10-04T19:56:29-05:00","caller":"cdnsd/main.go:72","msg":"starting DNS listener on :8053"}
{"level":"info","timestamp":"2023-10-04T19:56:35-05:00","caller":"dns/dns.go:43","msg":"query: name: foo.bar.com., type: A, class: IN"}
{"level":"debug","timestamp":"2023-10-04T19:56:35-05:00","caller":"dns/dns.go:185","msg":"querying 103.196.38.40:53: NS foo.bar.com."}
{"level":"debug","timestamp":"2023-10-04T19:56:35-05:00","caller":"dns/dns.go:185","msg":"querying 103.196.38.40:53: NS bar.com."}
{"level":"debug","timestamp":"2023-10-04T19:56:35-05:00","caller":"dns/dns.go:185","msg":"querying 103.196.38.40:53: A luke.ns.cloudflare.com."}
{"level":"debug","timestamp":"2023-10-04T19:56:35-05:00","caller":"dns/dns.go:185","msg":"querying 103.196.38.40:53: A pam.ns.cloudflare.com."}
{"level":"info","timestamp":"2023-10-04T19:56:38-05:00","caller":"dns/dns.go:43","msg":"query: name: foo.dolphin.cardano., type: A, class: IN"}

$ dig @localhost -p 8053 foo.dolphin.cardano

; <<>> DiG 9.18.12-0ubuntu0.22.04.3-Ubuntu <<>> @localhost -p 8053 foo.dolphin.cardano
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 17593
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 2
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;foo.dolphin.cardano.           IN      A

;; AUTHORITY SECTION:
dolphin.cardano.        999     IN      NS      ns2.dolphin.cardano.
dolphin.cardano.        999     IN      NS      ns1.dolphin.cardano.

;; ADDITIONAL SECTION:
ns2.dolphin.cardano.    999     IN      A       42.42.31.31
ns1.dolphin.cardano.    999     IN      A       0.0.0.0

;; Query time: 0 msec
;; SERVER: 127.0.0.1#8053(localhost) (UDP)
;; WHEN: Wed Oct 04 19:56:38 CDT 2023
;; MSG SIZE  rcvd: 203

$ dig @localhost -p 8053 foo.bar.com

; <<>> DiG 9.18.12-0ubuntu0.22.04.3-Ubuntu <<>> @localhost -p 8053 foo.bar.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 41495
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 2, ADDITIONAL: 6
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;foo.bar.com.                   IN      A

;; AUTHORITY SECTION:
bar.com.                999     IN      NS      luke.ns.cloudflare.com.
bar.com.                999     IN      NS      pam.ns.cloudflare.com.

;; ADDITIONAL SECTION:
luke.ns.cloudflare.com. 999     IN      A       173.245.59.200
luke.ns.cloudflare.com. 999     IN      A       172.64.33.200
luke.ns.cloudflare.com. 999     IN      A       108.162.193.200
pam.ns.cloudflare.com.  999     IN      A       108.162.192.138
pam.ns.cloudflare.com.  999     IN      A       173.245.58.138
pam.ns.cloudflare.com.  999     IN      A       172.64.32.138

;; Query time: 256 msec
;; SERVER: 127.0.0.1#8053(localhost) (UDP)
;; WHEN: Wed Oct 04 19:56:35 CDT 2023
;; MSG SIZE  rcvd: 339