Re: speedup COPY TO for partitioned table.

hi.

rebased and polished patch attached, test case added.
However there is a case (the following) where
``COPY(partitioned_table)`` is much slower
(around 25% in some cases) than ``COPY (select * from partitioned_table)``.

If the partition attribute order is not the same as the partitioned table,
then for each output row, we need to create a template TupleTableSlot
and call execute_attr_map_slot,
i didn't find a work around to reduce the inefficiency.

Since the master doesn't have ``COPY(partitioned_table)``,
I guess this slowness case is allowed?

----------- the follow case is far slower than ``COPY(select * From pp) TO ``
drop table if exists pp;
CREATE TABLE pp (id INT, val int ) PARTITION BY RANGE (id);
create table pp_1 (val int, id int);
create table pp_2 (val int, id int);
ALTER TABLE pp ATTACH PARTITION pp_1 FOR VALUES FROM (1) TO (5);
ALTER TABLE pp ATTACH PARTITION pp_2 FOR VALUES FROM (5) TO (10);
insert into pp select g, 10 + g from generate_series(1,9) g;
copy pp to stdout(header);