Add a method for clearing all retained messages at the server.

Build/MQTTnet.nuspec

@@ -11,7 +11,9 @@
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker).</description>
     <releaseNotes>* [Core] Added all factory methods to the factory interface.
+* [Core] Fixed an issue with cancellation token handling (thanks to @acrabb).
 * [Server] Added a new overload for configuring the ASP.net integration (thanks to @JanEggers).
+* [Server] Added a method for clearing all retained messages.
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2018</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags> 

Source/MQTTnet/Server/IMqttServer.cs

@@ -23,5 +23,7 @@ namespace MQTTnet.Server
 
         Task StartAsync(IMqttServerOptions options);
         Task StopAsync();
+
+        Task ClearRetainedMessagesAsync();
     }
 }

Source/MQTTnet/Server/MqttRetainedMessagesManager.cs

@@ -84,6 +84,16 @@ namespace MQTTnet.Server
             return retainedMessages;
         }
 
+        public Task ClearMessagesAsync()
+        {
+            lock (_messages)
+            {
+                _messages.Clear();
+            }
+
+            return _options.Storage.SaveRetainedMessagesAsync(new List<MqttApplicationMessage>());
+        }
+
         private async Task HandleMessageInternalAsync(string clientId, MqttApplicationMessage applicationMessage)
         {
             var saveIsRequired = false;

Source/MQTTnet/Server/MqttServer.cs

@@ -129,6 +129,11 @@ namespace MQTTnet.Server
             }
         }
 
+        public Task ClearRetainedMessagesAsync()
+        {
+            return _retainedMessagesManager?.ClearMessagesAsync();
+        }
+
         internal void OnClientConnected(string clientId)
         {
             _logger.Info("Client '{0}': Connected.", clientId);