Added IMqttRetainedMessagesManager

* Added interface IMqttRetainedMessagesManager to allow for different
retained messages manager implementations. The interface copies almost
exactly the current MqttRetainedMessagesManager implementation.

* Added IMqttRetainedMessagesManager.Start() to configure the provided
IMqttRetainedMessagesManager outside the constructor. This method returns
Task because some implementations that use external storage may be slow on
initialization (e.g., connect to a database).

* Modified MqttRetainedMessagesManager to implement new interface.

Source/MQTTnet/Server/MqttRetainedMessagesManager.cs

@@ -7,20 +7,21 @@ using MQTTnet.Internal;
 
 namespace MQTTnet.Server
 {
-    public class MqttRetainedMessagesManager
+    public class MqttRetainedMessagesManager : IMqttRetainedMessagesManager
     {
         private readonly byte[] _emptyArray = new byte[0];
         private readonly AsyncLock _messagesLock = new AsyncLock();
         private readonly Dictionary<string, MqttApplicationMessage> _messages = new Dictionary<string, MqttApplicationMessage>();
 
-        private readonly IMqttNetChildLogger _logger;
-        private readonly IMqttServerOptions _options;
+        private IMqttNetChildLogger _logger;
+        private IMqttServerOptions _options;
 
-        public MqttRetainedMessagesManager(IMqttServerOptions options, IMqttNetChildLogger logger)
+        public Task Start(IMqttServerOptions options, IMqttNetChildLogger logger)
         {
             if (logger == null) throw new ArgumentNullException(nameof(logger));
             _logger = logger.CreateChildLogger(nameof(MqttRetainedMessagesManager));
             _options = options ?? throw new ArgumentNullException(nameof(options));
+            return Task.CompletedTask;
         }
 
         public async Task LoadMessagesAsync()
@@ -128,7 +129,7 @@ namespace MQTTnet.Server
                     break;
                 }
             }
-            
+
             return matchingRetainedMessages;
         }