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/messageids.go

@@ -0,0 +1,79 @@
+/*
+ * Copyright (c) 2013 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:
+ *    Seth Hoenig
+ *    Allan Stockdill-Mander
+ *    Mike Robertson
+ */
+
+package mqtt
+
+import (
+	"fmt"
+	"sync"
+)
+
+// MId is 16 bit message id as specified by the MQTT spec.
+// In general, these values should not be depended upon by
+// the client application.
+type MId uint16
+
+type messageIds struct {
+	sync.RWMutex
+	index map[uint16]Token
+}
+
+const (
+	midMin uint16 = 1
+	midMax uint16 = 65535
+)
+
+func (mids *messageIds) cleanUp() {
+	mids.Lock()
+	for _, token := range mids.index {
+		switch t := token.(type) {
+		case *PublishToken:
+			t.err = fmt.Errorf("Connection lost before Publish completed")
+		case *SubscribeToken:
+			t.err = fmt.Errorf("Connection lost before Subscribe completed")
+		case *UnsubscribeToken:
+			t.err = fmt.Errorf("Connection lost before Unsubscribe completed")
+		}
+		token.flowComplete()
+	}
+	mids.index = make(map[uint16]Token)
+	mids.Unlock()
+}
+
+func (mids *messageIds) freeID(id uint16) {
+	mids.Lock()
+	delete(mids.index, id)
+	mids.Unlock()
+}
+
+func (mids *messageIds) getID(t Token) uint16 {
+	mids.Lock()
+	defer mids.Unlock()
+	for i := midMin; i < midMax; i++ {
+		if _, ok := mids.index[i]; !ok {
+			mids.index[i] = t
+			return i
+		}
+	}
+	return 0
+}
+
+func (mids *messageIds) getToken(id uint16) Token {
+	mids.RLock()
+	defer mids.RUnlock()
+	if token, ok := mids.index[id]; ok {
+		return token
+	}
+	return nil
+}