pgsql: Fix crash in autoprewarm.

Fix crash in autoprewarm.

Commit abb0b4fc03 moved the shared state for autoprewarm to a
dynamic shared memory (DSM) segment, but it left apw_detach_shmem()
in the on_shmem_exit callback list for the autoprewarm leader
process. This is a problem because shmem_exit() detaches all the
DSM segments prior to calling the on_shmem_exit callbacks, thus
producing segfaults in the exit path for the autoprewarm leader
process.

To fix, move apw_detach_shmem() to the before_shmem_exit callback
list. This commit also adds a check to pg_prewarm's test that the
server shut down normally. It might be worth making this a common
check for all shutdowns in TAP tests, but that is left as a future
exercise.

Reported-by: Andres Freund
Reviewed-by: Andres Freund, Álvaro Herrera
Discussion: https://postgr.es/m/20240122204117.swton324xcoodnyi%40awork3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/bcd5b4bcbefc8b99be3678cf9c6c990a0c28f68e

Modified Files
--------------
contrib/pg_prewarm/autoprewarm.c | 10 ++++++++--
contrib/pg_prewarm/t/001_basic.pl | 6 ++++++
2 files changed, 14 insertions(+), 2 deletions(-)