Reload configuration more frequently in apply worker.

Hi,

Currently, the main loop of apply worker looks like below[1]. Since there are
two loops, the inner loop will keep receiving and applying message from
publisher until no more message left. The worker only reloads the configuration in
the outer loop. This means if the publisher keeps sending messages (it could
keep sending multiple transactions), the apply worker won't get a chance to
update the GUCs.

[1]
for(;;) /* outer loop */
{
    for(;;) /* inner loop */
    {
        len = walrcv_receive()
        if (len == 0)
            break;
        ...
        apply change
    }

    ...
    if (ConfigReloadPending)
    {
        ConfigReloadPending = false;
        ProcessConfigFile(PGC_SIGHUP);
    }
    ...
}

I think it would be better that the apply worker can reflect user's
configuration changes sooner. To achieve this, we can add one more
ProcessConfigFile() call in the inner loop. Attach the patch for the same. What
do you think ?

BTW, I saw one BF failure[2] (it's very rare and only happened once in 4
months) which I think is due to the low frequent reload in apply worker.

The attached tap test shows how the failure happened.

The test use streaming parallel mode and change logical_replication_mode to
immediate, we expect serialization to happen in the test. To reproduce the failure
easier, we need to add a sleep(1s) in the inner loop of apply worker so
that the apply worker won't be able to consume all messages quickly and will be
busy in the inner loop. Then the attached test will fail because the leader
apply didn't reload the configuration, thus serialization didn't happen.

[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=mamba&dt=2023-05-12%2008%3A05%3A41

Best Regards,
Hou zj