all: implement policy failover rcodes

While at it, ensure that config is validated, and fixing a bug related
to reuse ctx between multiple upstreams resolving.

config_test.go

@@ -69,6 +69,7 @@ func TestConfigValidation(t *testing.T) {
 		{"invalid listener port", invalidListenerPort(t), true},
 		{"os upstream", configWithOsUpstream(t), false},
 		{"invalid rules", configWithInvalidRules(t), true},
+		{"invalid dns rcodes", configWithInvalidRcodes(t), true},
 	}
 
 	for _, tc := range tests {
@@ -155,6 +156,16 @@ func configWithInvalidRules(t *testing.T) *ctrld.Config {
 	return cfg
 }
 
+func configWithInvalidRcodes(t *testing.T) *ctrld.Config {
+	cfg := defaultConfig(t)
+	cfg.Listener["0"].Policy = &ctrld.ListenerPolicyConfig{
+		Name:           "Policy with invalid Rcodes",
+		Networks:       []ctrld.Rule{{"*.com": []string{"upstream.0"}}},
+		FailoverRcodes: []string{"foo"},
+	}
+	return cfg
+}
+
 func TestUpstreamConfig_Init(t *testing.T) {
 	tests := []struct {
 		name     string