Refactor subscribe and unsubscribe to support MQTTv5.

Source/MQTTnet/Client/Receiving/IMqttApplicationMessageHandler.cs

@@ -0,0 +1,9 @@
+using System.Threading.Tasks;
+
+namespace MQTTnet.Client.Receiving
+{
+    public interface IMqttApplicationMessageHandler
+    {
+        Task HandleApplicationMessageAsync(MqttApplicationMessageHandlerContext context);
+    }
+}

Source/MQTTnet/Client/Receiving/MqttApplicationMessageHandlerContext.cs

@@ -0,0 +1,15 @@
+namespace MQTTnet.Client.Receiving
+{
+    public class MqttApplicationMessageHandlerContext
+    {
+        public MqttApplicationMessageHandlerContext(string senderClientId, MqttApplicationMessage applicationMessage)
+        {
+            SenderClientId = senderClientId;
+            ApplicationMessage = applicationMessage;
+        }
+
+        public string SenderClientId { get; }
+
+        public MqttApplicationMessage ApplicationMessage { get; }
+    }
+}

Source/MQTTnet/Client/Receiving/MqttApplicationMessageHandlerDelegate.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Client.Receiving
+{
+    public class MqttApplicationMessageHandlerDelegate : IMqttApplicationMessageHandler
+    {
+        private readonly Func<MqttApplicationMessageHandlerContext, Task> _handler;
+
+        public MqttApplicationMessageHandlerDelegate(Action<MqttApplicationMessageHandlerContext> handler)
+        {
+            if (handler == null) throw new ArgumentNullException(nameof(handler));
+
+            _handler = context =>
+            {
+                handler(context);
+                return Task.FromResult(0);
+            };
+        }
+
+        public MqttApplicationMessageHandlerDelegate(Func<MqttApplicationMessageHandlerContext, Task> handler)
+        {
+            _handler = handler ?? throw new ArgumentNullException(nameof(handler));
+        }
+
+        public Task HandleApplicationMessageAsync(MqttApplicationMessageHandlerContext context)
+        {
+            return _handler(context);
+        }
+    }
+}