This adds *most* of the tests from SkyDNS, things lacking is
the stubzone checking, groups and the trim prefix. These
will be added in subsequent PRs and in separate test files.
This middleware acts in the same way as SkyDNS. We might add options
to allow it to be behave different, but for now it will suffice.
A Corefile like:
.:1053 {
etcd miek.nl
proxy . 8.8.8.8:53
}
will perform lookup in etcd and proxy everything not miek.nl to Google
for further resolution.
The internal etcd forwarding *also* uses the proxy infrastructure,
meaning you get health check and such for (almost) free