Bring etcd support for bucket DNS federation

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

vendor/github.com/miekg/dns/udp_other.go

@@ -0,0 +1,17 @@
+// +build !linux,!plan9
+
+package dns
+
+import (
+	"net"
+	"syscall"
+)
+
+// These do nothing. See udp_linux.go for an example of how to implement this.
+
+// We tried to adhire to some kind of naming scheme.
+
+func setUDPSocketOptions4(conn *net.UDPConn) error                 { return nil }
+func setUDPSocketOptions6(conn *net.UDPConn) error                 { return nil }
+func getUDPSocketOptions6Only(conn *net.UDPConn) (bool, error)     { return false, nil }
+func getUDPSocketName(conn *net.UDPConn) (syscall.Sockaddr, error) { return nil, nil }