Support outgoing zone transfers

These can be enabled by adding "transfer out" to the Corefile. Without
it no AXFR is allowed.

For now only AXFR and no IXFR. No TSIG and no ACLs.

middleware/file/notify.go

@@ -0,0 +1,8 @@
+package file
+
+// Notify sends notifies to the configured remotes. It will try up to three times
+// before giving up on a specific remote.
+func Notify(remotes []string) error {
+	return nil
+
+}