Fix stubzone retention (#198)

Make the receiver a pointer so that the uptdateStubZones map update will
retain the stubzones found, unlike the current case where the update
will be applied and then promptly forgotten, because it is working on a
copy.

Add test/etcd_test.go to test a large part of the code. This didn't
catch the chaos middleware hack though. The chaos middleware zones are
now *not* automatically added. You have to take care of that by yourself
(docs updates).

When using debug queries and falling through to the next middleware in
etcd, restore the original (with o-o.debug) query before passing it on.

middleware/etcd/stub_test.go

@@ -19,6 +19,7 @@ func TestStubLookup(t *testing.T) {
 		defer delete(t, etc, serv.Key)
 	}
 	etc.updateStubZones()
+	defer func() { etc.Stubmap = nil }()
 
 	for _, tc := range dnsTestCasesStub {
 		m := tc.Msg()