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/SessionTests.cs

@@ -627,6 +627,30 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.EditData
             Assert.Throws<InvalidOperationException>(() => s.RevertCell(0, 0));
         }
 
+        [Fact]
+        public async Task RevertCellRowRevert()
+        {
+            // Setup:
+            // ... Create a session with a proper query and metadata
+            EditSession s = await GetBasicSession();
+
+            // ... Add a edit that we will say the edit was reverted if we update a cell
+            var mockEdit = new Mock<RowEditBase>();
+            mockEdit.Setup(e => e.RevertCell(It.IsAny<int>()))
+                .Returns(new EditRevertCellResult {IsRowDirty = false});
+            s.EditCache[0] = mockEdit.Object;
+
+            // If: I update a cell that will return an implicit revert
+            s.RevertCell(0, 0);
+
+            // Then:
+            // ... Set cell should have been called on the mock update once
+            mockEdit.Verify(e => e.RevertCell(0), Times.Once);
+
+            // ... The mock update should no longer be in the edit cache
+            Assert.Empty(s.EditCache);
+        }
+
         #endregion
 
         #region Update Cell Tests
@@ -653,7 +677,8 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.EditData
 
             // ... Add a mock edit to the edit cache to cause the .TryAdd to fail
             var mockEdit = new Mock<RowEditBase>();
-            mockEdit.Setup(e => e.SetCell(It.IsAny<int>(), It.IsAny<string>()));
+            mockEdit.Setup(e => e.SetCell(It.IsAny<int>(), It.IsAny<string>()))
+                .Returns(new EditUpdateCellResult {IsRowDirty = true});
             s.EditCache[0] = mockEdit.Object;
 
             // If: I update a cell on a row that already has a pending edit
@@ -681,6 +706,32 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.EditData
             Assert.IsType<RowUpdate>(s.EditCache[0]);
         }
 
+        [Fact]
+        public async Task UpdateCellRowRevert()
+        {
+            // Setup:
+            // ... Create a session with a proper query and metadata
+            EditSession s = await GetBasicSession();
+
+            // ... Add a edit that we will say the edit was reverted if we update a cell
+            var mockEdit = new Mock<RowEditBase>();
+            mockEdit.Setup(e => e.SetCell(It.IsAny<int>(), It.IsAny<string>()))
+                .Returns(new EditUpdateCellResult {IsRowDirty = false});
+            s.EditCache[0] = mockEdit.Object;
+
+            // If: I update a cell that will return an implicit revert
+            s.UpdateCell(0, 0, null);
+
+            // Then:
+            // ... Set cell should have been called on the mock update once
+            mockEdit.Verify(e => e.SetCell(0, null), Times.Once);
+
+            // ... The mock update should no longer be in the edit cache
+            Assert.Empty(s.EditCache);
+        }
+
+
+
         #endregion
 
         #region SubSet Tests