Add wait-for-lsn process-exit cleanup callback

WaitLSNCleanup() was called from ProcKill(), but not from
AuxiliaryProcKill(), even though xlogwait.c sizes its shared memory to
include NUM_AUXILIARY_PROCS and thus accepts calls from auxiliary
processes.  Such a process exiting while waiting would leave its entry in
the heap.

Register an on_shmem_exit callback lazily before a process enters a
wait-for-lsn heap instead, as suggested by Noah: that keeps the cleanup
local to xlogwait.c and makes it harder to miss a caller that needs it,
rather than having to remember every process-kill path.

Reported-by: Noah Misch <[email protected]>
Author: Xuneng Zhou <[email protected]>
Reviewed-by: Alexander Korotkov <[email protected]>
Discussion: https://postgr.es/m/20260706012642.f9.noahmisch%40microsoft.com
Discussion: 
https://postgr.es/m/CABPTF7UtW_cAa%3DQh4RDfKiUqu3pJJE22ai9tbWJVERbeRyssLw%40mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f994d2b71ebca8258f7c68be310077588aa62059

Modified Files
--------------
src/backend/access/transam/xlogwait.c | 38 +++++++++++++++++++++++++++++++++++
src/backend/storage/lmgr/proc.c       |  6 ------
2 files changed, 38 insertions(+), 6 deletions(-)

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