nat4-nat4 punch (#388)

this patch optimize the udp hole punch logic:

1. allow start punch hole before stun test complete.
2. add lock to symmetric punch, avoid conflict between concurrent hole punching task.
3. support punching hole for predictable nat4-nat4.
4. make backoff of retry reasonable

easytier/src/instance/instance.rs

@@ -161,7 +161,7 @@ impl Instance {
             DirectConnectorManager::new(global_ctx.clone(), peer_manager.clone());
         direct_conn_manager.run();
 
-        let udp_hole_puncher = UdpHolePunchConnector::new(global_ctx.clone(), peer_manager.clone());
+        let udp_hole_puncher = UdpHolePunchConnector::new(peer_manager.clone());
 
         let peer_center = Arc::new(PeerCenterInstance::new(peer_manager.clone()));