Use WaitLatch() instead of pg_usleep() at end-of-vacuum truncation

This has the advantage to make a process more responsive when the
postmaster dies, even if the wait time was rather limited as there was
only a 50ms timeout here.  Another advantage of this change is for
monitoring, as we gain a new wait event for the end-of-vacuum
truncation.

Author: Bharath Rupireddy
Reviewed-by: Aleksander Alekseev, Thomas Munro, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACU4AdPCq6NLfcA-ZGwX7pPCK5FgEj-CAU0xCKzkASSy_A@mail.gmail.com

src/backend/access/heap/vacuumlazy.c

@@ -3236,7 +3236,11 @@ lazy_truncate_heap(LVRelState *vacrel)
 				return;
 			}
 
-			pg_usleep(VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL * 1000L);
+			(void) WaitLatch(MyLatch,
+							 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
+							 VACUUM_TRUNCATE_LOCK_WAIT_INTERVAL,
+							 WAIT_EVENT_VACUUM_TRUNCATE);
+			ResetLatch(MyLatch);
 		}
 
 		/*

src/backend/utils/activity/wait_event.c

@@ -485,6 +485,9 @@ pgstat_get_wait_timeout(WaitEventTimeout w)
 		case WAIT_EVENT_VACUUM_DELAY:
 			event_name = "VacuumDelay";
 			break;
+		case WAIT_EVENT_VACUUM_TRUNCATE:
+			event_name = "VacuumTruncate";
+			break;
 			/* no default case, so that compiler will warn */
 	}