pgsql: Add wait-for-lsn process-exit cleanup callback

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 <noah(at)leadboat(dot)com>
Author: Xuneng Zhou <xunengzhou(at)gmail(dot)com>
Reviewed-by: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
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
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REL_19_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7864a14c4370b11cac74bcdf4cae5bb6a6ae867b

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