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/README.md

@@ -0,0 +1,34 @@
+# secondary
+
+`secondary` enables serving a zone retrieved from a primary server.
+
+## Syntax
+
+~~~
+secondary [zones...]
+~~~
+
+* `zones` zones it should be authoritative for. If empty, the zones from the configuration block
+    are used. Not that with an remote address to *get* the zone the above is not that useful.
+
+A working syntax would be:
+
+~~~
+secondary [zones...] {
+    transfer from address
+    [transfer to address]
+}
+~~~
+
+* `transfer from` tell from which address to fetch the zone. It can be specified multiple time,
+    if one does not work another will be tried.
+* `transfer to` can be enabled to allow this secondary zone to be transfered again.
+
+## Examples
+
+~~~
+secondary [zones...] {
+    transfer from 10.0.1.1
+    transfer from 10.1.2.1
+}
+~~~