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.

vendor/github.com/eclipse/paho.mqtt.golang/packets/unsubscribe.go

@@ -0,0 +1,55 @@
+package packets
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+//UnsubscribePacket is an internal representation of the fields of the
+//Unsubscribe MQTT packet
+type UnsubscribePacket struct {
+	FixedHeader
+	MessageID uint16
+	Topics    []string
+}
+
+func (u *UnsubscribePacket) String() string {
+	str := fmt.Sprintf("%s", u.FixedHeader)
+	str += " "
+	str += fmt.Sprintf("MessageID: %d", u.MessageID)
+	return str
+}
+
+func (u *UnsubscribePacket) Write(w io.Writer) error {
+	var body bytes.Buffer
+	var err error
+	body.Write(encodeUint16(u.MessageID))
+	for _, topic := range u.Topics {
+		body.Write(encodeString(topic))
+	}
+	u.FixedHeader.RemainingLength = body.Len()
+	packet := u.FixedHeader.pack()
+	packet.Write(body.Bytes())
+	_, err = packet.WriteTo(w)
+
+	return err
+}
+
+//Unpack decodes the details of a ControlPacket after the fixed
+//header has been read
+func (u *UnsubscribePacket) Unpack(b io.Reader) error {
+	u.MessageID = decodeUint16(b)
+	var topic string
+	for topic = decodeString(b); topic != ""; topic = decodeString(b) {
+		u.Topics = append(u.Topics, topic)
+	}
+
+	return nil
+}
+
+//Details returns a Details struct containing the Qos and
+//MessageID of this ControlPacket
+func (u *UnsubscribePacket) Details() Details {
+	return Details{Qos: 1, MessageID: u.MessageID}
+}