More options for disconnect (#1557)

* Expose more options in disconnect options.

* Expose more options in disconnect API.

* Refactor code.

* Expose user properties in disconnected event.

* Expose more disconnect options in server.

* Fix build errors

* Add new samples.

* Fix client cleanup

* Fix merge conflicts

Source/MQTTnet/Internal/TaskExtensions.cs

@@ -2,8 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using MQTTnet.Diagnostics;
+using System;
 using System.Threading.Tasks;
+using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Internal
 {
@@ -11,7 +12,8 @@ namespace MQTTnet.Internal
     {
         public static void RunInBackground(this Task task, MqttNetSourceLogger logger = null)
         {
-            task?.ContinueWith(t =>
+            task?.ContinueWith(
+                t =>
                 {
                     // Consume the exception first so that we get no exception regarding the not observed exception.
                     var exception = t.Exception;
@@ -19,5 +21,41 @@ namespace MQTTnet.Internal
                 },
                 TaskContinuationOptions.OnlyOnFaulted);
         }
+
+        public static async Task WaitAsync(this Task task, Task sender, MqttNetSourceLogger logger)
+        {
+            if (logger == null)
+            {
+                throw new ArgumentNullException(nameof(logger));
+            }
+
+            if (task == null)
+            {
+                return;
+            }
+
+            if (task == sender)
+            {
+                // Return here to avoid deadlocks, but first any eventual exception in the task
+                // must be handled to avoid not getting an unhandled task exception
+                if (!task.IsFaulted)
+                {
+                    return;
+                }
+
+                // By accessing the Exception property the exception is considered handled and will
+                // not result in an unhandled task exception later by the finalizer
+                logger.Warning(task.Exception, "Error while waiting for background task.");
+                return;
+            }
+
+            try
+            {
+                await task.ConfigureAwait(false);
+            }
+            catch (OperationCanceledException)
+            {
+            }
+        }
     }
-}
+}