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/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 */
 	}