Fix anonymous column names

The problem is that we misunderstood the anonymous column name generation logic. It was found out that Tarantool generates the names of UNION and EXCEPT nodes independently, i.e.

select COLUMN_1 as a from (values (null)) where false
union
select COLUMN_1 as a from (values (null)) where false;

select COLUMN_1 as a from (values (null)) where false
except
select COLUMN_1 as a from (values (null)) where false;

We expected that the column names would be generated for the whole plan tree, not for the subtree (it was wrong):

select COLUMN_1 as a from (values (null)) where false
union
select COLUMN_2 as a from (values (null)) where false;