Add async handlers for connect and disconnect, too

.. this allows a client to properly async publish on connect.

Source/MQTTnet.Extensions.ManagedClient/IManagedMqttClient.cs

@@ -13,7 +13,12 @@ namespace MQTTnet.Extensions.ManagedClient
         int PendingApplicationMessagesCount { get; }
         IManagedMqttClientOptions Options { get; }
 
+        IMqttClientConnectedHandler ConnectedHandler { get; set; }
+        [Obsolete("Use ConnectedHandler instead.")]
         event EventHandler<MqttClientConnectedEventArgs> Connected;
+
+        IMqttClientDisconnectedHandler DisconnectedHandler { get; set; }
+        [Obsolete("Use DisconnectedHandler instead.")]
         event EventHandler<MqttClientDisconnectedEventArgs> Disconnected;
 
         event EventHandler<ApplicationMessageProcessedEventArgs> ApplicationMessageProcessed;
@@ -21,7 +26,7 @@ namespace MQTTnet.Extensions.ManagedClient
 
         event EventHandler<MqttManagedProcessFailedEventArgs> ConnectingFailed;
         event EventHandler<MqttManagedProcessFailedEventArgs> SynchronizingSubscriptionsFailed;
-        
+
         Task StartAsync(IManagedMqttClientOptions options);
         Task StopAsync();
 

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClient.cs

@@ -4,6 +4,8 @@ using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Client;
+using MQTTnet.Client.Connecting;
+using MQTTnet.Client.Disconnecting;
 using MQTTnet.Client.Publishing;
 using MQTTnet.Client.Receiving;
 using MQTTnet.Diagnostics;
@@ -50,9 +52,13 @@ namespace MQTTnet.Extensions.ManagedClient
         public int PendingApplicationMessagesCount => _messageQueue.Count;
         public IManagedMqttClientOptions Options { get; private set; }
 
+        public IMqttClientConnectedHandler ConnectedHandler { get; set; }
         public event EventHandler<MqttClientConnectedEventArgs> Connected;
+
+        public IMqttClientDisconnectedHandler DisconnectedHandler { get; set; }
         public event EventHandler<MqttClientDisconnectedEventArgs> Disconnected;
 
+
         public IMqttApplicationMessageHandler ReceivedApplicationMessageHandler
         {
             get => _mqttClient.ReceivedApplicationMessageHandler;
@@ -424,13 +430,13 @@ namespace MQTTnet.Extensions.ManagedClient
         private Task OnDisconnected(MqttClientDisconnectedEventArgs eventArgs)
         {
             Disconnected?.Invoke(this, eventArgs);
-            return Task.FromResult(0);
+            return DisconnectedHandler?.HandleDisconnectedAsync(eventArgs);
         }
 
         private Task OnConnected(MqttClientConnectedEventArgs eventArgs)
         {
             Connected?.Invoke(this, eventArgs);
-            return Task.FromResult(0);
+            return ConnectedHandler?.HandleConnectedAsync(eventArgs);
         }
 
         private void StartPublishing()

Source/MQTTnet.Extensions.ManagedClient/ManagedMqttClientExtensions.cs

@@ -2,6 +2,8 @@
 using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Client.Connecting;
+using MQTTnet.Client.Disconnecting;
 using MQTTnet.Client.Publishing;
 using MQTTnet.Client.Receiving;
 using MQTTnet.Protocol;
@@ -10,6 +12,30 @@ namespace MQTTnet.Extensions.ManagedClient
 {
     public static class ManagedMqttClientExtensions
     {
+        public static IManagedMqttClient UseConnectedHandler(this IManagedMqttClient client, Func<MqttClientConnectedEventArgs, Task> handler)
+        {
+            if (handler == null)
+            {
+                client.ConnectedHandler = null;
+                return client;
+            }
+
+            client.ConnectedHandler = new MqttClientConnectedHandlerDelegate(handler);
+            return client;
+        }
+
+        public static IManagedMqttClient UseDisconnectedHandler(this IManagedMqttClient client, Func<MqttClientDisconnectedEventArgs, Task> handler)
+        {
+            if (handler == null)
+            {
+                client.DisconnectedHandler = null;
+                return client;
+            }
+
+            client.DisconnectedHandler = new MqttClientDisconnectedHandlerDelegate(handler);
+            return client;
+        }
+
         public static IManagedMqttClient UseReceivedApplicationMessageHandler(this IManagedMqttClient client, Func<MqttApplicationMessageHandlerContext, Task> handler)
         {
             if (handler == null)