flatten timeline pdus iterations; increase concurrency

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

src/api/server/send.rs

@@ -135,6 +135,7 @@ async fn handle_pdus(
 			.rooms
 			.event_handler
 			.handle_incoming_pdu(origin, &room_id, &event_id, value, true)
+			.boxed()
 			.await
 			.map(|_| ());