Use ordinals to force stable TopN heap sort

[max-hoffman]

Re: dolthub/dolt#5640

... ORDER BY <expr> LIMIT 1 and ... ORDER BY <expr> LIMIT 1 OFFSET 1 both use TopN in a way that surprises users expecting stable output orders.

For example, the second query returning the first row is within the SQL spec but not user friendly:

-- setup
create table xy (x int primary key, y int);
insert into xy values (1,0),(2,0),(3,0),(4,0);
-- expect first row
select * from xy order by y asc limit 1;
+---+------+
| x | y    |
+---+------+
| 0 |    1 |
+---+------+
-- expect second row
select * from xy order by y asc limit 1 offset 1;
+---+------+
| x | y    |
+---+------+
| 0 |    1 |
+---+------+

This PR adds ordinals to rows in the TopN heap to differentiate insertion order in sort-groupings.

[PavelSafronov]

Nit: rename childLen to childSchemaLen?

[max-hoffman]

added

[reltuk]

LGTM!