fixed compilation

Source/MQTTnet/Implementations/PlatformAbstractionLayer.cs

@@ -88,5 +88,17 @@ namespace MQTTnet.Implementations
 #endif
         }
 
+        public static Task CompletedTask
+        {
+            get 
+            {
+#if NET452 
+                return Task.FromResult(0);
+#else
+                return Task.CompletedTask;
+#endif
+            }
+        }
+
     }
 }

Source/MQTTnet/Server/MqttRetainedMessagesManager.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
+using MQTTnet.Implementations;
 using MQTTnet.Internal;
 
 namespace MQTTnet.Server
@@ -21,7 +22,7 @@ namespace MQTTnet.Server
             if (logger == null) throw new ArgumentNullException(nameof(logger));
             _logger = logger.CreateChildLogger(nameof(MqttRetainedMessagesManager));
             _options = options ?? throw new ArgumentNullException(nameof(options));
-            return Task.CompletedTask;
+            return PlatformAbstractionLayer.CompletedTask;
         }
 
         public async Task LoadMessagesAsync()