Allow AuthNS to host mulitple zones

[pinheadmz]

I'm pretty sure this is something BIND can do - if I want to run a service where users can delegate their NS to me and I am authoritative over multiple zones.

With bns, an AuthNS can only have one zone:

https://github.com/chjj/bns/blob/03b0ea5e7587774c0c3ad363f094a069acc1e60b/lib/server/auth.js#L20

I may explore a solution that serves an array of zones