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/config-old.go

@@ -449,3 +449,22 @@ type serverConfigV17 struct {
 	// Notification queue configuration.
 	Notify *notifier `json:"notify"`
 }
+
+// serverConfigV18 server configuration version '18' which is like
+// version '17' except it adds support for "deliveryMode" parameter in
+// the AMQP notification target.
+type serverConfigV18 struct {
+	sync.RWMutex
+	Version string `json:"version"`
+
+	// S3 API configuration.
+	Credential credential  `json:"credential"`
+	Region     string      `json:"region"`
+	Browser    BrowserFlag `json:"browser"`
+
+	// Additional error logging configuration.
+	Logger *loggers `json:"logger"`
+
+	// Notification queue configuration.
+	Notify *notifier `json:"notify"`
+}