We should remove this API as a simplification of the memory transfer model.
It was used in various places to manage lifetime of BufferHandles, but those can be replaced with attaching the BufferHandle + MarkFlushResources().
It is also used to work around the fact that some memory is never returned from ExecNodes and is freed in Close(). Part of the solution is probably to add a way to attach all resources to a RowBatch before Close() and during Reset().
|Remove MarkNeedsDeepCopy() from UnionNode||Open||Unassigned|