Implicit Revert Row Cleanup Logic (#297)

This change enhances the way that edit/updateCell and edit/revertCell operations are performed. 

## **THE API BREAKING CHANGES**:
* edit/updateCell now returns an EditCell (a DbCellValue with a dirty flag) and a row dirty flag.
* edit/revertCell now returns an EditCell (a DbCellValue with a dirty flag) and a row dirty flag.

If by setting the value of a cell via edit/updateCell the row no longer has any edits (an "implicit revert"), the entire row's edit will be removed from the cache. Additionally, if by requesting edit/revert all the pending edits for a row are removed, the entire row's edit will be removed from the cache. This will prevent issues where committing will generate an invalid script because it has no pending changes.

* Adding EditCell class
Returning EditCell with EditUpdateCellResult

* Adding code that will remove a row update if the row is clean after a cell update

* Adding code that will return an EditCell and row dirty flag when a cell is reverted.
If the row is reverted by the cell revert, the pending update will be removed

* Comments for edit cell

* Changes as per pull request comments

src/Microsoft.SqlTools.ServiceLayer/EditData/Contracts/EditRevertCellRequest.cs

@@ -7,14 +7,19 @@ using Microsoft.SqlTools.Hosting.Protocol.Contracts;
 
 namespace Microsoft.SqlTools.ServiceLayer.EditData.Contracts
 {
+    /// <summary>
+    /// Parameters for the cell revert request
+    /// </summary>
     public class EditRevertCellParams : RowOperationParams
     {
         public int ColumnId { get; set; }
     }
 
-    public class EditRevertCellResult
+    /// <summary>
+    /// Parameters to return upon successful revert of the cell
+    /// </summary>
+    public class EditRevertCellResult : EditCellResult
     {
-        public string NewValue { get; set; }
     }
 
     public class EditRevertCellRequest