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/event-notifier.go

@@ -609,6 +609,25 @@ func loadAllQueueTargets() (map[string]*logrus.Logger, error) {
 		}
 	}
 
+	// Load all mqtt targets, initialize their respective loggers.
+	for accountID, mqttN := range serverConfig.Notify.GetMQTT() {
+		if !mqttN.Enable {
+			continue
+		}
+
+		if queueARN, err := addQueueTarget(queueTargets, accountID, queueTypeMQTT, newMQTTNotify); err != nil {
+			if _, ok := err.(net.Error); ok {
+				err = &net.OpError{
+					Op:  "Connecting to " + queueARN,
+					Net: "tcp",
+					Err: err,
+				}
+			}
+
+			return nil, err
+		}
+	}
+
 	// Load all nats targets, initialize their respective loggers.
 	for accountID, natsN := range serverConfig.Notify.GetNATS() {
 		if !natsN.Enable {