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);
+    }
+}

MediaBrowser.Controller/Net/IHttpResultFactory.cs

@@ -28,7 +28,7 @@ namespace MediaBrowser.Controller.Net
         /// <returns>System.Object.</returns>
         object GetResult(object content, string contentType, IDictionary<string,string> responseHeaders = null);
 
-        object GetAsyncStreamWriter(Func<Stream,Task> streamWriter, IDictionary<string, string> responseHeaders = null);
+        object GetAsyncStreamWriter(IAsyncStreamSource streamSource);
 
         /// <summary>
         /// Gets the optimized result.