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/tree.go

@@ -528,6 +528,8 @@ func (n *Node) floor(rr dns.RR) *Node {
 	return n
 }
 
+// TODO(successor, predecessor)
+
 // Ceil returns the smallest value equal to or greater than the query q according to q.Compare().
 func (t *Tree) Ceil(rr dns.RR) *Elem {
 	if t.Root == nil {