Use a WaitLatch for vacuum/autovacuum sleeping

Instead of using pg_usleep() in vacuum_delay_point(), use a WaitLatch.
This has the advantage that we will realize if the postmaster has been
killed since the last time we decided to sleep while vacuuming.

Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/CAFh8B=kcdk8k-Y21RfXPu5dX=bgPqJ8TC3p_qxR_ygdBS=JN5w@mail.gmail.com

src/backend/commands/vacuum.c

@@ -2080,9 +2080,11 @@ vacuum_delay_point(void)
 		if (msec > VacuumCostDelay * 4)
 			msec = VacuumCostDelay * 4;
 
-		pgstat_report_wait_start(WAIT_EVENT_VACUUM_DELAY);
-		pg_usleep((long) (msec * 1000));
-		pgstat_report_wait_end();
+		(void) WaitLatch(MyLatch,
+						 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
+						 msec,
+						 WAIT_EVENT_VACUUM_DELAY);
+		ResetLatch(MyLatch);
 
 		VacuumCostBalance = 0;