Not including gateway non-delay when calculating total packet delay (#507)

common/nymsphinx/acknowledgements/src/surb_ack.rs

@@ -72,7 +72,8 @@ impl SURBAck {
             .build_packet(surb_ack_payload, &route, &destination, &delays)
             .unwrap();
 
-        let expected_total_delay = delays.iter().sum();
+        // in our case, the last hop is a gateway that does NOT do any delays
+        let expected_total_delay = delays.iter().take(delays.len() - 1).sum();
         let first_hop_address =
             NymNodeRoutingAddress::try_from(route.first().unwrap().address).unwrap();
 

common/nymsphinx/src/preparer/mod.rs

@@ -331,7 +331,8 @@ where
         Ok(PreparedFragment {
             // the round-trip delay is the sum of delays of all hops on the forward route as
             // well as the total delay of the ack packet.
-            total_delay: delays.iter().sum::<Delay>() + ack_delay,
+            // note that the last hop of the packet is a gateway that does not do any delays
+            total_delay: delays.iter().take(delays.len() - 1).sum::<Delay>() + ack_delay,
             mix_packet: MixPacket::new(first_hop_address, sphinx_packet, self.mode),
         })
     }