Add support for MQTT server as a notification target (#4474)

This implementation is similar to AMQP notifications:

* Notifications are published on a single topic as a JSON feed
* Topic is configurable, as is the QoS. Uses the paho.mqtt.golang
  library for the mqtt connection, and supports connections over tcp
  and websockets, with optional secure tls support.
* Additionally the minio server configuration has been bumped up
  so mqtt configuration can be added.
* Configuration migration code is added with tests.

MQTT is an ISO standard M2M/IoT messaging protocol and was
originally designed for applications for limited bandwidth
networks. Today it's use is growing in the IoT space.

cmd/notifiers.go

@@ -30,6 +30,8 @@ const (
 
 	// Static string indicating queue type 'amqp'.
 	queueTypeAMQP = "amqp"
+	// Static string indicating queue type 'mqtt'.
+	queueTypeMQTT = "mqtt"
 	// Static string indicating queue type 'nats'.
 	queueTypeNATS = "nats"
 	// Static string indicating queue type 'elasticsearch'.
@@ -80,6 +82,25 @@ func isAMQPQueue(sqsArn arnSQS) bool {
 	return true
 }
 
+// Returns true if mqttARN is for an MQTT queue.
+func isMQTTQueue(sqsArn arnSQS) bool {
+	if sqsArn.Type != queueTypeMQTT {
+		return false
+	}
+	mqttL := serverConfig.Notify.GetMQTTByID(sqsArn.AccountID)
+	if !mqttL.Enable {
+		return false
+	}
+	// Connect to mqtt server to validate.
+	mqttC, err := dialMQTT(mqttL)
+	if err != nil {
+		errorIf(err, "Unable to connect to mqtt service. %#v", mqttL)
+		return false
+	}
+	defer mqttC.Client.Disconnect(250)
+	return true
+}
+
 // Returns true if natsArn is for an NATS queue.
 func isNATSQueue(sqsArn arnSQS) bool {
 	if sqsArn.Type != queueTypeNATS {