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/publish.go

@@ -0,0 +1,80 @@
+package packets
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+//PublishPacket is an internal representation of the fields of the
+//Publish MQTT packet
+type PublishPacket struct {
+	FixedHeader
+	TopicName string
+	MessageID uint16
+	Payload   []byte
+}
+
+func (p *PublishPacket) String() string {
+	str := fmt.Sprintf("%s", p.FixedHeader)
+	str += " "
+	str += fmt.Sprintf("topicName: %s MessageID: %d", p.TopicName, p.MessageID)
+	str += " "
+	str += fmt.Sprintf("payload: %s", string(p.Payload))
+	return str
+}
+
+func (p *PublishPacket) Write(w io.Writer) error {
+	var body bytes.Buffer
+	var err error
+
+	body.Write(encodeString(p.TopicName))
+	if p.Qos > 0 {
+		body.Write(encodeUint16(p.MessageID))
+	}
+	p.FixedHeader.RemainingLength = body.Len() + len(p.Payload)
+	packet := p.FixedHeader.pack()
+	packet.Write(body.Bytes())
+	packet.Write(p.Payload)
+	_, err = w.Write(packet.Bytes())
+
+	return err
+}
+
+//Unpack decodes the details of a ControlPacket after the fixed
+//header has been read
+func (p *PublishPacket) Unpack(b io.Reader) error {
+	var payloadLength = p.FixedHeader.RemainingLength
+	p.TopicName = decodeString(b)
+	if p.Qos > 0 {
+		p.MessageID = decodeUint16(b)
+		payloadLength -= len(p.TopicName) + 4
+	} else {
+		payloadLength -= len(p.TopicName) + 2
+	}
+	if payloadLength < 0 {
+		return fmt.Errorf("Error upacking publish, payload length < 0")
+	}
+	p.Payload = make([]byte, payloadLength)
+	_, err := b.Read(p.Payload)
+
+	return err
+}
+
+//Copy creates a new PublishPacket with the same topic and payload
+//but an empty fixed header, useful for when you want to deliver
+//a message with different properties such as Qos but the same
+//content
+func (p *PublishPacket) Copy() *PublishPacket {
+	newP := NewControlPacket(Publish).(*PublishPacket)
+	newP.TopicName = p.TopicName
+	newP.Payload = p.Payload
+
+	return newP
+}
+
+//Details returns a Details struct containing the Qos and
+//MessageID of this ControlPacket
+func (p *PublishPacket) Details() Details {
+	return Details{Qos: p.Qos, MessageID: p.MessageID}
+}