Do not disable PTO when packets outstanding == 0 for initial/handshake

See recovery 6.2.2.1, the client needs to keep sending packets until it
receives HandshakeDone, even if all previous PTOs are acked and
in_flight_outstanding drops to zero.

neqo-transport/src/recovery.rs

@@ -312,8 +312,9 @@ impl LossRecoverySpace {
         if p.ack_eliciting() {
             debug_assert!(self.in_flight_outstanding > 0);
             self.in_flight_outstanding -= 1;
-            if self.in_flight_outstanding == 0 {
-                qtrace!("remove_packet outstanding == 0");
+            if self.space == PNSpace::ApplicationData && self.in_flight_outstanding == 0 {
+                // See above comments; keep PTO armed for Initial/Handshake even
+                // if no outstanding packets.
                 self.pto_base_time = None;
             }
         }