Schedule ShutdownXLOG() in single user mode using before_shmem_exit().

Schedule ShutdownXLOG() in single user mode using 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().  There does not seem to be any
reason to do ShutdownXLOG() via on_shmem_exit(), so change it.

Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20210405092914.mmxqe7j56lsjfsej@alap3.anarazel.de
Discussion: https://postgr.es/m/20210803023612.iziacxk5syn2r4ut@alap3.anarazel.de