Add multiple domain support

Do not merge this (yet), this is not the version with per-domain handlers.

This PR adds to **SkyDNS** the ability to be authoritative for more than a single domain. Please see the commit messages for more details.

- [ ] one domain, one handler
- [ ] masquerade the answering NS (domains can have their own names for "ns.dns.skydns.local")
- [ ] move the query cache out of the handlers (optimal: before the mux)
- [ ] remove syscalls for time (a goroutine which is called every X ms will suffice; for cache/TTL see Redis)
- [ ] cross-domain recursion — how is that signed? (www.skynet.test CNAME srv1.region4.some.tld → IP)
I would call it `domain` just like in `sign`