pg_reload_conf() synchronously

In an internal conversation it was seen that for some tests that want
to enforce a behaviour, a behaviour that is controlled by a GUC, we
_need_ to perform a sleep for an arbitrary amount of time.
Alternatively, executing the rest of the test on a new connection also
helps to get the expected behaviour. Following is a sample snippet of
such a test.

ALTER SYSTEM SET param TO value;
SELECT pg_reload_conf();
-- Either pg_sleep(0.1) or \connect here for next command to behave as expected.
ALTER ROLE ... PASSWORD '...';

This is because of the fact that the SIGHUP, sent to Postmaster by
this backend, takes some time to get back to the issuing backend.

Although a pg_sleep() call works to alleviate the pain in most cases,
it does not provide any certainty. Following the Principle Of Least
Astonishment, we want a command that loads the configuration
_synchronously_, so that the subsequent commands from the client can
be confident that the requisite parameter changes have taken effect.

The attached patch makes the pg_reload_conf() function set
ConfigReloadPending to true, which will force the postgres main
command-processing loop to process and apply config changes _before_
executing the next command.

The only downside I can think of this approach is that it _might_
cause the issuing backend to process the config file twice; once after
it has processed the current command, and another time when the SIGHUP
signal comes from the Postmaster. If the SIGHUP signal from the
Postmaster comes before the end of current command, then the issuing
backend will process the config only once, as before the patch. In any
case, this extra processing of the config seems harmless, and the
benefit outweighs the extra processing done.

The alternate methods that I considered (see branch reload_my_conf at
[2]) were to either implement the SQL command RELOAD CONFIGURATION [
FOR ALL ], or to create an overloaded version of function
pg_reload_conf(). But both those approaches had much more significant
downsides, like additional GRANTs, etc.

There have been issues identified in the past (see [1]) that possibly
may be alleviated by this approach of applying config changes
synchronously.

[1]: https://www.postgresql.org/message-id/2138662.1623460441%40sss.pgh.pa.us
[2]: https://github.com/gurjeet/postgres/tree/reload_my_conf

Best regards,
Gurjeet
http://Gurje.et