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/tree/all.go

@@ -0,0 +1,21 @@
+package tree
+
+// All traverses tree and returns all elements
+func (t *Tree) All() []*Elem {
+	if t.Root == nil {
+		return nil
+	}
+	found := t.Root.all(nil)
+	return found
+}
+
+func (n *Node) all(found []*Elem) []*Elem {
+	if n.Left != nil {
+		found = n.Left.all(found)
+	}
+	found = append(found, n.Elem)
+	if n.Right != nil {
+		found = n.Right.all(found)
+	}
+	return found
+}