[multistage] Support limit properly

[walterddr]

trafficstars

Currently select * from tbl limit 10 returns a unimplemented exception because limit is considered as an Order By with fetch top 10.

since we do not order with limit normally we should:

• [ ] relax the planner condition to allow randomly return 10 rows
• [ ] truncate the final result to the limit
• [ ] propagate limit down to table scan

[walterddr]

fetch unorder is supported but we should still limit the number of rows returned at final stage.