Add secondary support

Allow specifying a primary server and retrieve the zone's content.

Add tests and an Expired bool to zone struct, to stop server zones

that are expired. The zone is retrieved on Startup, no updates of

changed content are done. We also don't respond to notifies yet.

middleware/secondary/secondary.go

@@ -0,0 +1,7 @@
+package secondary
+
+import "github.com/miekg/coredns/middleware/file"
+
+type Secondary struct {
+	file.File
+}