Bring etcd support for bucket DNS federation

- Supports centralized `config.json`
- Supports centralized `bucket` service records
  for client lookups
- implement a new proxy forwarder

docs/federation/lookup/Corefile.example

@@ -0,0 +1,11 @@
+. {
+  etcd churchofminio.com {
+      endpoint http://localhost:2379 http://localhost:4001
+      upstream /etc/resolv.conf
+  }
+  debug
+  prometheus
+  cache 160 churchofminio.com
+  loadbalance
+  proxy . /etc/resolv.conf
+}