Remove duplicate table scan in logical apply worker and code refactoring

Hi,

When reviewing the code in logical/worker.c, I noticed that when applying a
cross-partition update action, it scans the old partition twice.
I am attaching the patch 0001 to remove this duplicate table scan.

The test shows that it brings noticeable improvement:

Steps
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Pub:
create table tab (a int not null, b int);
alter table tab replica identity full;
insert into tab select 1,generate_series(1, 1000000, 1);

Sub:
create table tab (a int not null, b int) partition by range (b);
create table tab_1 partition of tab for values from (minvalue) to (5000000);
create table tab_2 partition of tab for values from (5000000) to (maxvalue);
alter table tab replica identity full;


Test query:
update tab set b = 6000000 where b > 999900; -- UPDATE 100

Results (The time spent by apply worker to apply the all the UPDATEs):
Before    14s
After    7s
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Apart from above, I found there are quite a few duplicate codes related to partition
handling(e.g. apply_handle_tuple_routing), so I tried to extract some
common logic to simplify the codes. Please see 0002 for this refactoring.

Best Regards,
Hou Zhijie