Reorganize solution structure.

Source/MQTTnet.Extensions.ManagedClient/IManagedMqttClientStorage.cs

@@ -0,0 +1,12 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace MQTTnet.Extensions.ManagedClient
+{
+    public interface IManagedMqttClientStorage
+    {
+        Task SaveQueuedMessagesAsync(IList<ManagedMqttApplicationMessage> messages);
+
+        Task<IList<ManagedMqttApplicationMessage>> LoadQueuedMessagesAsync();
+    }
+}