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/token.go

@@ -0,0 +1,157 @@
+/*
+ * Copyright (c) 2014 IBM Corp.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *    Allan Stockdill-Mander
+ */
+
+package mqtt
+
+import (
+	"sync"
+	"time"
+
+	"github.com/eclipse/paho.mqtt.golang/packets"
+)
+
+//PacketAndToken is a struct that contains both a ControlPacket and a
+//Token. This struct is passed via channels between the client interface
+//code and the underlying code responsible for sending and receiving
+//MQTT messages.
+type PacketAndToken struct {
+	p packets.ControlPacket
+	t Token
+}
+
+//Token defines the interface for the tokens used to indicate when
+//actions have completed.
+type Token interface {
+	Wait() bool
+	WaitTimeout(time.Duration) bool
+	flowComplete()
+	Error() error
+}
+
+type baseToken struct {
+	m        sync.RWMutex
+	complete chan struct{}
+	ready    bool
+	err      error
+}
+
+// Wait will wait indefinitely for the Token to complete, ie the Publish
+// to be sent and confirmed receipt from the broker
+func (b *baseToken) Wait() bool {
+	b.m.Lock()
+	defer b.m.Unlock()
+	if !b.ready {
+		<-b.complete
+		b.ready = true
+	}
+	return b.ready
+}
+
+// WaitTimeout takes a time in ms to wait for the flow associated with the
+// Token to complete, returns true if it returned before the timeout or
+// returns false if the timeout occurred. In the case of a timeout the Token
+// does not have an error set in case the caller wishes to wait again
+func (b *baseToken) WaitTimeout(d time.Duration) bool {
+	b.m.Lock()
+	defer b.m.Unlock()
+	if !b.ready {
+		select {
+		case <-b.complete:
+			b.ready = true
+		case <-time.After(d):
+		}
+	}
+	return b.ready
+}
+
+func (b *baseToken) flowComplete() {
+	close(b.complete)
+}
+
+func (b *baseToken) Error() error {
+	b.m.RLock()
+	defer b.m.RUnlock()
+	return b.err
+}
+
+func newToken(tType byte) Token {
+	switch tType {
+	case packets.Connect:
+		return &ConnectToken{baseToken: baseToken{complete: make(chan struct{})}}
+	case packets.Subscribe:
+		return &SubscribeToken{baseToken: baseToken{complete: make(chan struct{})}, subResult: make(map[string]byte)}
+	case packets.Publish:
+		return &PublishToken{baseToken: baseToken{complete: make(chan struct{})}}
+	case packets.Unsubscribe:
+		return &UnsubscribeToken{baseToken: baseToken{complete: make(chan struct{})}}
+	case packets.Disconnect:
+		return &DisconnectToken{baseToken: baseToken{complete: make(chan struct{})}}
+	}
+	return nil
+}
+
+//ConnectToken is an extension of Token containing the extra fields
+//required to provide information about calls to Connect()
+type ConnectToken struct {
+	baseToken
+	returnCode byte
+}
+
+//ReturnCode returns the acknowlegement code in the connack sent
+//in response to a Connect()
+func (c *ConnectToken) ReturnCode() byte {
+	c.m.RLock()
+	defer c.m.RUnlock()
+	return c.returnCode
+}
+
+//PublishToken is an extension of Token containing the extra fields
+//required to provide information about calls to Publish()
+type PublishToken struct {
+	baseToken
+	messageID uint16
+}
+
+//MessageID returns the MQTT message ID that was assigned to the
+//Publish packet when it was sent to the broker
+func (p *PublishToken) MessageID() uint16 {
+	return p.messageID
+}
+
+//SubscribeToken is an extension of Token containing the extra fields
+//required to provide information about calls to Subscribe()
+type SubscribeToken struct {
+	baseToken
+	subs      []string
+	subResult map[string]byte
+}
+
+//Result returns a map of topics that were subscribed to along with
+//the matching return code from the broker. This is either the Qos
+//value of the subscription or an error code.
+func (s *SubscribeToken) Result() map[string]byte {
+	s.m.RLock()
+	defer s.m.RUnlock()
+	return s.subResult
+}
+
+//UnsubscribeToken is an extension of Token containing the extra fields
+//required to provide information about calls to Unsubscribe()
+type UnsubscribeToken struct {
+	baseToken
+}
+
+//DisconnectToken is an extension of Token containing the extra fields
+//required to provide information about calls to Disconnect()
+type DisconnectToken struct {
+	baseToken
+}