pgstat: Schedule per-backend pgstat shutdown via before_shmem_exit().

pgstat: Schedule per-backend pgstat shutdown via before_shmem_exit().

Previously on_shmem_exit() was used. The upcoming shared memory stats patch
uses DSM segments to store stats, which can not be used after the
dsm_backend_shutdown() call in shmem_exit().

The preceding commits were required to permit this change. This commit is
split off the shared memory stats patch to make it easier to isolate problems
caused by the ordering changes rather than the much larger changes in where
stats are stored.

Author: Andres Freund <andres@anarazel.de>
Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20210405092914.mmxqe7j56lsjfsej@alap3.anarazel.de
Discussion: https://postgr.es/m/20210802164124.ufo5buo4apl6yuvs@alap3.anarazel.de
Discussion: https://postgr.es/m/20210803023612.iziacxk5syn2r4ut@alap3.anarazel.de