Added support for persisted retained messages.

Source/MQTTnet.Server/Configuration/RetainedApplicationMessagesModel.cs

@@ -0,0 +1,11 @@
+namespace MQTTnet.Server.Configuration
+{
+    public class RetainedApplicationMessagesModel
+    {
+        public bool Persist { get; set; } = false;
+
+        public int WriteInterval { get; set; } = 10;
+
+        public string Filename { get; set; } = "RetainedApplicationMessages.json";
+    }
+}

Source/MQTTnet.Server/Configuration/SettingsModel.cs

@@ -5,10 +5,6 @@
     /// </summary>
     public class SettingsModel
     {
-        public SettingsModel()
-        {
-        }
-
         /// <summary>
         /// Set default connection timeout in seconds
         /// </summary>
@@ -43,5 +39,10 @@
         /// Set limit for max pending messages per client
         /// </summary>
         public int MaxPendingMessagesPerClient { get; set; } = 250;
+
+        /// <summary>
+        /// The settings for retained messages.
+        /// </summary>
+        public RetainedApplicationMessagesModel RetainedApplicationMessages { get; set; } = new RetainedApplicationMessagesModel();
     }
 }