Include Internal Row ID with DbCellValue (#308)

Update to include the Row ID with a DbCellValue. This will be super useful to us for the purposes of managing individual rows via slick grid.

Although this changes the API for edit/subset and query/subset, it is effectively backwards compatible since it's adding a parameter, not removing anything.

* New DbCellValue has an internal Row ID

* Adding unit tests

src/Microsoft.SqlTools.ServiceLayer/QueryExecution/Contracts/DbCellValue.cs

@@ -27,6 +27,12 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts
         /// </summary>
         internal object RawObject { get; set; }
 
+        /// <summary>
+        /// The internal ID for the row. Should be used when directly referencing the row for edit
+        /// or other purposes.
+        /// </summary>
+        public long RowId { get; set; }
+
         /// <summary>
         /// Copies the values of this DbCellValue into another DbCellValue (or child object)
         /// </summary>
@@ -38,6 +44,7 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts
             other.DisplayValue = DisplayValue;
             other.IsNull = IsNull;
             other.RawObject = RawObject;
+            other.RowId = RowId;
         }
     }
 }