Revert "Make zone.Signatures a map of a map"
Revert "Make zone.Signatures a map of a map"

This reverts commit f4f936e2e86516dd29918a7c5a0212e86a2dcc7d.

jdef/dns,hugozhu/dns,rolandshoemaker/dns,curtiszimmerman/dns,josephyzhou/dns,michaelharo/dns,asergeyev/dns,letsencrypt/dns,tonnerre/golang-dns,bragr/dns,jmhodges/dns,janstk/dns,skyportsystems/dns,miekg/dns,tempbottle/dns,modulexcite/dns,bboreham/dns,ErikDubbelboer/dns,tsenart/dns,skynet/dns,benbearchen/dns,cloudflare/dns,ym/go-dns,Git-Host/dns,ezoic/dns,AlexCheong/dns,flebarbe/dns,FiloSottile/dns,Psiphon-Inc/dns