Make DNS queries case insensitive

[johnmaguire]

Convert hostnames to lowercase during map insertion and lookup.

Fixes #776.

Manually tested...

INFO[0000] Starting DNS responder                        dnsListener="0.0.0.0:53"
INFO[0005] Handshake message received                    certName=NoDe1 fingerprint=71544988eed070f0599ce8627918479be60f493b2c729d5abb85fb025e9d0330 handshake="map[stage:1 style:ix_psk0]" initiatorIndex=3501997041 issuer=0e1f5f42920c4e24c12496c4
d0f199ecbe0fff92bda4edac352ebd6c2eb7ce84 remoteIndex=0 responderIndex=0 udpAddr="127.0.0.1:58582" vpnIp=192.168.128.2
INFO[0005] Handshake message sent                        certName=NoDe1 fingerprint=71544988eed070f0599ce8627918479be60f493b2c729d5abb85fb025e9d0330 handshake="map[stage:2 style:ix_psk0]" initiatorIndex=3501997041 issuer=0e1f5f42920c4e24c12496c4
d0f199ecbe0fff92bda4edac352ebd6c2eb7ce84 remoteIndex=0 responderIndex=743908020 sentCachedPackets=0 udpAddr="127.0.0.1:58582" vpnIp=192.168.128.2

❯ dig @127.0.0.01 nOdE1

; <<>> DiG 9.10.6 <<>> @127.0.0.01 nOdE1
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 20643
;; flags: qr rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;nOdE1.				IN	A

;; ANSWER SECTION:
nOdE1.			3600	IN	A	192.168.128.2

;; Query time: 9 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Dec 13 15:02:37 EST 2022
;; MSG SIZE  rcvd: 44

❯ dig @127.0.0.01 NODE1

; <<>> DiG 9.10.6 <<>> @127.0.0.01 NODE1
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 25154
;; flags: qr rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;NODE1.				IN	A

;; ANSWER SECTION:
NODE1.			3600	IN	A	192.168.128.2

;; Query time: 8 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Dec 13 15:02:39 EST 2022
;; MSG SIZE  rcvd: 44

[jasikpark]

Will require a docs update:

https://github.com/DefinedNet/nebula-docs/blob/74427e8a44aa73c1a945c0bdb7adedd222ceefc8/docs/guides/using-lighthouse-dns/index.mdx?plain=1#L113-L116

[jasikpark]

lgtm

ig this could increase the number of duplicate hostnames if someone has a network w/ names that are only differing in case, though this is arguably a bugfix, it makes sense as a breaking change