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

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);