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/connack.go

@@ -0,0 +1,51 @@
+package packets
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+//ConnackPacket is an internal representation of the fields of the
+//Connack MQTT packet
+type ConnackPacket struct {
+	FixedHeader
+	SessionPresent bool
+	ReturnCode     byte
+}
+
+func (ca *ConnackPacket) String() string {
+	str := fmt.Sprintf("%s", ca.FixedHeader)
+	str += " "
+	str += fmt.Sprintf("sessionpresent: %t returncode: %d", ca.SessionPresent, ca.ReturnCode)
+	return str
+}
+
+func (ca *ConnackPacket) Write(w io.Writer) error {
+	var body bytes.Buffer
+	var err error
+
+	body.WriteByte(boolToByte(ca.SessionPresent))
+	body.WriteByte(ca.ReturnCode)
+	ca.FixedHeader.RemainingLength = 2
+	packet := ca.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 (ca *ConnackPacket) Unpack(b io.Reader) error {
+	ca.SessionPresent = 1&decodeByte(b) > 0
+	ca.ReturnCode = decodeByte(b)
+
+	return nil
+}
+
+//Details returns a Details struct containing the Qos and
+//MessageID of this ControlPacket
+func (ca *ConnackPacket) Details() Details {
+	return Details{Qos: 0, MessageID: 0}
+}