Make save result async (#107)

* Make save results asynchronous

* Prevent write share of file

* Lock objects in stages

* Create Save result objects

* refactor and write rows in batches

* CHange batchSize from test value

* Remove await in handler

* Removing the file reader as a member of the resultset

* Change Dispose to wait for save

* Change concurrentBag

* PascalCase variables

* Modify function signature and tests

* Safe file methods

* refactor ResultSets to Ilist and remove ToList

* Change dictionary key and prevent add to saveTasks during dispose

* Simplify row concatenation

* Fix prevent add

* Fix prevent add

* Add methods to expose saveTasks and isBeingDisposed

src/Microsoft.SqlTools.ServiceLayer/QueryExecution/FileUtils.cs

@@ -0,0 +1,46 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+using System;
+using System.IO;
+namespace Microsoft.SqlTools.ServiceLayer.QueryExecution
+{
+    internal static class FileUtils
+    {
+        /// <summary>
+        /// Checks if file exists and swallows exceptions, if any
+        /// </summary>
+        /// <param name="path"> path of the file</param>
+        /// <returns></returns>
+        internal static bool SafeFileExists(string path)
+        {
+            try
+            {
+                return File.Exists(path);
+            }
+            catch (Exception)
+            {
+                // Swallow exception
+                return false;
+            }
+        }
+
+        /// <summary>
+        /// Deletes a file and swallows exceptions, if any
+        /// </summary>
+        /// <param name="path"></param>
+        internal static void SafeFileDelete(string path)
+        {
+            try
+            {
+                File.Delete(path);
+            }
+            catch (Exception)
+            {
+                // Swallow exception, do nothing
+            }
+        }
+
+    }
+}