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

test/Microsoft.SqlTools.ServiceLayer.UnitTests/EditData/RowCreateTests.cs

@@ -263,15 +263,17 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.EditData
             EditUpdateCellResult eucr = rc.SetCell(0, "1");
 
             // Then:
-            // ... The returned value should not have corrections
-            Assert.False(eucr.HasCorrections);
-            Assert.Null(eucr.NewValue);
-            
-            // ... The set value is not null
-            Assert.False(eucr.IsNull);
+            // ... The returned value should be equal to what we provided
+            Assert.NotNull(eucr);
+            Assert.NotNull(eucr.Cell);
+            Assert.Equal("1", eucr.Cell.DisplayValue);
+            Assert.False(eucr.Cell.IsNull);
 
-            // ... The result is not an implicit revert
-            Assert.False(eucr.IsRevert);
+            // ... The returned value should be dirty
+            Assert.NotNull(eucr.Cell.IsDirty);
+
+            // ... The row should still be dirty
+            Assert.True(eucr.IsRowDirty);
 
             // ... There should be a cell update in the cell list
             Assert.NotNull(rc.newCells[0]);
@@ -303,15 +305,17 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.EditData
             EditUpdateCellResult eucr = rc.SetCell(0, "1000");
 
             // Then:
-            // ... The returned value should have corrections
-            Assert.True(eucr.HasCorrections);
-            Assert.NotEmpty(eucr.NewValue);
+            // ... The returned value should be equal to what we provided
+            Assert.NotNull(eucr);
+            Assert.NotNull(eucr.Cell);
+            Assert.NotEqual("1000", eucr.Cell.DisplayValue);
+            Assert.False(eucr.Cell.IsNull);
 
-            // ... The set value is not null
-            Assert.False(eucr.IsNull);
+            // ... The returned value should be dirty
+            Assert.NotNull(eucr.Cell.IsDirty);
 
-            // ... The result is not an implicit revert
-            Assert.False(eucr.IsRevert);
+            // ... The row should still be dirty
+            Assert.True(eucr.IsRowDirty);
 
             // ... There should be a cell update in the cell list
             Assert.NotNull(rc.newCells[0]);
@@ -327,15 +331,17 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.EditData
             EditUpdateCellResult eucr = rc.SetCell(0, "NULL");
 
             // Then:
-            // ... The returned value should not have corrections
-            Assert.False(eucr.HasCorrections);
-            Assert.Null(eucr.NewValue);
+            // ... The returned value should be equal to what we provided
+            Assert.NotNull(eucr);
+            Assert.NotNull(eucr.Cell);
+            Assert.NotEmpty(eucr.Cell.DisplayValue);
+            Assert.True(eucr.Cell.IsNull);
 
-            // ... The set value is null
-            Assert.True(eucr.IsNull);
+            // ... The returned value should be dirty
+            Assert.NotNull(eucr.Cell.IsDirty);
 
-            // ... The result is not an implicit revert
-            Assert.False(eucr.IsRevert);
+            // ... The row should still be dirty
+            Assert.True(eucr.IsRowDirty);
 
             // ... There should be a cell update in the cell list
             Assert.NotNull(rc.newCells[0]);
@@ -362,11 +368,18 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.EditData
             RowCreate rc = await GetStandardRowCreate();
 
             // If: I attempt to revert a cell that has not been set
-            string result = rc.RevertCell(0);
+            EditRevertCellResult result = rc.RevertCell(0);
 
-            // Then: We should get null back
+            // Then:
+            // ... We should get a result back
+            Assert.NotNull(result);
+
+            // ... We should get a null cell back
             // @TODO: Check for a default value when we support it
-            Assert.Null(result);
+            Assert.Null(result.Cell);
+
+            // ... The row should be dirty
+            Assert.True(result.IsRowDirty);
 
             // ... The cell should no longer be set
             Assert.Null(rc.newCells[0]);
@@ -380,11 +393,18 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.EditData
             rc.SetCell(0, "1");
 
             // If: I attempt to revert a cell that was set
-            string result = rc.RevertCell(0);
+            EditRevertCellResult result = rc.RevertCell(0);
 
             // Then:
-            // ... We should get null back
-            Assert.Null(result);
+            // ... We should get a result back
+            Assert.NotNull(result);
+
+            // ... We should get a null cell back
+            // @TODO: Check for a default value when we support it
+            Assert.Null(result.Cell);
+
+            // ... The row should be dirty
+            Assert.True(result.IsRowDirty);
 
             // ... The cell should no longer be set
             Assert.Null(rc.newCells[0]);