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/EditCell.cs

@@ -0,0 +1,39 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
+
+namespace Microsoft.SqlTools.ServiceLayer.EditData.Contracts
+{
+    /// <summary>
+    /// Cell that wraps info from <see cref="DbCellValue"/> for edit purposes
+    /// </summary>
+    public class EditCell : DbCellValue
+    {
+        /// <summary>
+        /// Default, parameterless constructor to make sure that JSON serializing is happy
+        /// </summary>
+        public EditCell() {}
+
+        /// <summary>
+        /// Constructs a new EditCell based on a DbCellValue
+        /// </summary>
+        /// <param name="dbCellValue">The DbCellValue that will be enhanced</param>
+        /// <param name="isDirty">Whether or not the edit cell is dirty</param>
+        public EditCell(DbCellValue dbCellValue, bool isDirty)
+        {
+            IsDirty = isDirty;
+
+            DisplayValue = dbCellValue.DisplayValue;
+            IsNull = dbCellValue.IsNull;
+            RawObject = dbCellValue.RawObject;
+        }
+
+        /// <summary>
+        /// Whether or not the cell is considered dirty
+        /// </summary>
+        public bool IsDirty { get; set; }
+    }
+}

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

@@ -0,0 +1,18 @@
+namespace Microsoft.SqlTools.ServiceLayer.EditData.Contracts
+{
+    /// <summary>
+    /// Parameters to return when a cell is updated in isolation
+    /// </summary>
+    public class EditCellResult
+    {
+        /// <summary>
+        /// The cell after the revert was applied
+        /// </summary>
+        public EditCell Cell { get; set; }
+
+        /// <summary>
+        /// Whether or not the row is dirty after the revert has been applied
+        /// </summary>
+        public bool IsRowDirty { get; set; }
+    }
+}

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

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

@@ -26,31 +26,8 @@ namespace Microsoft.SqlTools.ServiceLayer.EditData.Contracts
     /// <summary>
     /// Parameters to return upon successful update of the cell
     /// </summary>
-    public class EditUpdateCellResult
+    public class EditUpdateCellResult : EditCellResult
     {
-        /// <summary>
-        /// Whether or not the cell value was modified from the provided string.
-        /// If <c>true</c>, the client should replace the display value of the cell with the value
-        /// in <see cref="NewValue"/>
-        /// </summary>
-        public bool HasCorrections { get; set; }
-
-        /// <summary>
-        /// Whether or not the cell was reverted with the change.
-        /// If <c>true</c>, the client should unmark the cell as having an update and replace the
-        /// display value of the cell with the value in <see cref="NewValue"/>
-        /// </summary>
-        public bool IsRevert { get; set; }
-
-        /// <summary>
-        /// Whether or not the new value of the cell is null
-        /// </summary>
-        public bool IsNull { get; set; }
-
-        /// <summary>
-        /// The new string value of the cell
-        /// </summary>
-        public string NewValue { get; set; }
     }
 
     public class EditUpdateCellRequest