BUG #17537: Dynamically updating statement_timeout not affect the insert query right now.

The following bug has been logged on the website:

Bug reference:      17537
Logged by:          Parachute Parachute
Email address:      teng_wang13@163.com
PostgreSQL version: 14.2
Operating system:   Ubuntu 18.04
Description:

As per the manual,
statement_timeout (integer)
Abort any statement that takes more than the specified amount of time. A
value of zero (the default) disables the timeout. The timeout is measured
from the time a command arrives at the server until it is completed by the
server.
https://www.postgresql.org/docs/14/runtime-config-client.html

However, dynamically updating statement_timeout did not affect the insert
query right now.
I start up Postgres with statement_timeout set 0, which means disabling the
timeout. Then I insert 100000000 rows into a table, which finally takes
166498.487ms. During the insert operation, I consider the operation would
takes too much time. So I update statement_timeout to 10s in
postgresql.conf, and "select pg_reload_conf();" in another session to reload
configuration files.

I saw the logs showing the parameter was updated:
2022-07-03  [4229] LOG:  received SIGHUP, reloading configuration files
2022-07-03  [4229] LOG:  parameter "statement_timeout" changed to "1000"

However, the insert query did not been cancelled, and finally toke about
166s. I think that updating the timeout parameter should also affect the
operation in progress. Because an operation may sometimes take too long, but
it is not safe to interrupt it abruptly.

The scenario:

postgres=# show statement_timeout;
 statement_timeout 
-------------------
 0
(1 row)


postgres=# insert into tbl_test select
generate_series(1,100000000),md5(random()::text),clock_timestamp();

2022-07-03 03:25:10.470 UTC [4229] LOG:  received SIGHUP, reloading
configuration files
2022-07-03 03:25:10.471 UTC [4229] LOG:  parameter "statement_timeout"
changed to "1000"
INSERT 0 100000000
Time: 166498.487 ms (02:46.498)


In another session:(to update parameter "statement_timeout")
postgres=# select pg_reload_conf();
 pg_reload_conf 
----------------
 t
(1 row)

postgres=# show statement_timeout;
 statement_timeout 
-------------------
 10s
(1 row)