Async stream handling: Use interface instead of Func<Stream,Task>

No functional changes

MediaBrowser.Controller/Net/IAsyncStreamSource.cs

@@ -0,0 +1,18 @@
+using ServiceStack.Web;
+using System.IO;
+using System.Threading.Tasks;
+
+namespace MediaBrowser.Controller.Net
+{
+    /// <summary>
+    /// Interface IAsyncStreamSource
+    /// Enables asynchronous writing to http resonse streams
+    /// </summary>
+    public interface IAsyncStreamSource
+    {
+        /// <summary>
+        /// Asynchronously write to the response stream.
+        /// </summary>
+        Task WriteToAsync(Stream responseStream);
+    }
+}