Optimize Pdu prev_events and auth_events containers.

Signed-off-by: Jason Volk <jason@zemos.net>

src/api/client/state.rs

@@ -195,6 +195,7 @@ async fn send_state_event_for_key_helper(
 			room_id,
 			&state_lock,
 		)
+		.boxed()
 		.await?;
 
 	Ok(event_id)

src/api/client/sync/v3.rs

@@ -533,9 +533,9 @@ async fn handle_left_room(
 			unsigned: None,
 			// The following keys are dropped on conversion
 			room_id: room_id.clone(),
-			prev_events: vec![],
+			prev_events: Default::default(),
+			auth_events: Default::default(),
 			depth: uint!(1),
-			auth_events: vec![],
 			redacts: None,
 			hashes: EventHash::default(),
 			signatures: None,