aio: Fix configuration reload in IO workers.

aio: Fix configuration reload in IO workers.

method_worker.c installed SignalHandlerForConfigReload, but it failed to
actually process reload requests.  That hasn't yet produced any concrete
problem reports in terms of GUC changes it should have cared about in
v18, but it was inconsistent.

It did cause problems for a couple of patches in development that need
IO workers to react to ALTER SYSTEM + pg_reload_conf().  Fix extracted
from one of those patches.

Back-patch to 18.

Reported-by: Dmitry Dolgov <9erthalion6@gmail.com>
Discussion: http://postgr.es/m/sh5uqe4a4aqo5zkkpfy5fobe2rg2zzouctdjz7kou4t74c66ql%40yzpkxb7pgoxf