Fix LOCK_TIMEOUT handling in slotsync worker.

Fix LOCK_TIMEOUT handling in slotsync worker.

Previously, the slotsync worker relied on SIGINT for graceful shutdown
during promotion. However, SIGINT is also used by the LOCK_TIMEOUT handler
to cancel queries. Since the slotsync worker can lock catalog tables while
parsing libpq tuples, this overlap caused it to ignore LOCK_TIMEOUT
signals and potentially wait indefinitely on locks.

This patch replaces the slotsync worker's SIGINT handler with
StatementCancelHandler to correctly process query-cancel interrupts.
Additionally, the startup process now uses SIGUSR1 to signal the slotsync
worker to stop during promotion. The worker exits after detecting that the
shared memory flag stopSignaled is set.

Author: Hou Zhijie <houzj.fnst@fujitsu.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 17, here it was introduced
Discussion: https://postgr.es/m/TY4PR01MB169078F33846E9568412D878C94A2A@TY4PR01MB16907.jpnprd01.prod.outlook.com