pgsql: Fix LOCK_TIMEOUT handling during parallel apply.

Fix LOCK_TIMEOUT handling during parallel apply.

Previously, the parallel apply worker used SIGINT to receive a graceful
shutdown signal from the leader apply worker. However, SIGINT is also used
by the LOCK_TIMEOUT handler to trigger a query-cancel interrupt. This
overlap caused the parallel apply worker to miss LOCK_TIMEOUT signals,
leading to incorrect behavior during lock wait/contention.

This patch resolves the conflict by switching the graceful shutdown signal
from SIGINT to SIGUSR2.

Reported-by: Zane Duffield <duffieldzane@gmail.com>
Diagnosed-by: Zhijie Hou <houzj.fnst@fujitsu.com>
Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 16, where it was introduced
Discussion: https://postgr.es/m/CACMiCkXyC4au74kvE2g6Y=mCEF8X6r-Ne_ty4r7qWkUjRE4+oQ@mail.gmail.com

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/a54c7a1133a4338c09b435f7870152f305bbf3ad

Modified Files
--------------
src/backend/postmaster/interrupt.c                    |  5 ++---
src/backend/replication/logical/applyparallelworker.c | 17 ++++++++++++-----
src/backend/replication/logical/launcher.c            |  4 ++--
3 files changed, 16 insertions(+), 10 deletions(-)