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/pubcomp.go

@@ -0,0 +1,44 @@
+package packets
+
+import (
+	"fmt"
+	"io"
+)
+
+//PubcompPacket is an internal representation of the fields of the
+//Pubcomp MQTT packet
+type PubcompPacket struct {
+	FixedHeader
+	MessageID uint16
+}
+
+func (pc *PubcompPacket) String() string {
+	str := fmt.Sprintf("%s", pc.FixedHeader)
+	str += " "
+	str += fmt.Sprintf("MessageID: %d", pc.MessageID)
+	return str
+}
+
+func (pc *PubcompPacket) Write(w io.Writer) error {
+	var err error
+	pc.FixedHeader.RemainingLength = 2
+	packet := pc.FixedHeader.pack()
+	packet.Write(encodeUint16(pc.MessageID))
+	_, err = packet.WriteTo(w)
+
+	return err
+}
+
+//Unpack decodes the details of a ControlPacket after the fixed
+//header has been read
+func (pc *PubcompPacket) Unpack(b io.Reader) error {
+	pc.MessageID = decodeUint16(b)
+
+	return nil
+}
+
+//Details returns a Details struct containing the Qos and
+//MessageID of this ControlPacket
+func (pc *PubcompPacket) Details() Details {
+	return Details{Qos: pc.Qos, MessageID: pc.MessageID}
+}