Fix filebrowser to handle cancellation and multiple open/expand requests (#499)

* add cancel filebrowser request

* update expand request contract

* cleanup

* add queue

* add expand queue and update cancellation

* dispose cancelsource

* remove unnecessary field

* cleanup using directives

* address pr comment

* more changes

* change contract

src/Microsoft.SqlTools.ServiceLayer/FileBrowser/Contracts/FileBrowserOpenRequest.cs

@@ -26,6 +26,11 @@ namespace Microsoft.SqlTools.ServiceLayer.FileBrowser.Contracts
         /// File extension filter (e.g. *.bak)
         /// </summary>
         public string[] FileFilters;
+
+        /// <summary>
+        /// True if this is a request to change file filter
+        /// </summary>
+        public bool ChangeFilter;
     }
 
     /// <summary>