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/DataStorage/IFileStreamReader.cs

@@ -14,6 +14,6 @@ namespace Microsoft.SqlTools.ServiceLayer.QueryExecution.DataStorage
     /// </summary>
     public interface IFileStreamReader : IDisposable
     {
-        IList<DbCellValue> ReadRow(long offset, IEnumerable<DbColumnWrapper> columns);
+        IList<DbCellValue> ReadRow(long offset, long rowId, IEnumerable<DbColumnWrapper> columns);
     }
 }