The setting has no effect except during startup.  It's still nice to be
able to change it dynamically, which is expected to be pretty useful to
an admin following crash recovery when restarting the cluster is not so
appealing.

Per discussions following commits 2941138e and 61752afb.

Author: Justin Pryzby <pryzby@telsasoft.com>
Reviewed-by: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://postgr.es/m/20210529192321.GM2082%40telsasoft.com