Fix Route serialization round-trip

When the `max_total_routing_fee_msat` parameter was added to
`RouteParameters`, the serialization used `map` to get the max fee,
accidentally writing an `Option<Option<u64>>`, but then read it as
an `Option<u64>`. Thus, any `Route`s with a `route_params` written
will fail to be read back.

Luckily, this is an incredibly rarely-used bit of code, so only one
user managed to hit it.

lightning/src/routing/router.rs

@@ -526,7 +526,7 @@ impl Writeable for Route {
 			(1, self.route_params.as_ref().map(|p| &p.payment_params), option),
 			(2, blinded_tails, optional_vec),
 			(3, self.route_params.as_ref().map(|p| p.final_value_msat), option),
-			(5, self.route_params.as_ref().map(|p| p.max_total_routing_fee_msat), option),
+			(5, self.route_params.as_ref().and_then(|p| p.max_total_routing_fee_msat), option),
 		});
 		Ok(())
 	}

lightning/src/util/test_utils.rs

@@ -143,6 +143,7 @@ impl<'a> Router for TestRouter<'a> {
 		&self, payer: &PublicKey, params: &RouteParameters, first_hops: Option<&[&ChannelDetails]>,
 		inflight_htlcs: InFlightHtlcs
 	) -> Result<Route, msgs::LightningError> {
+		let route_res;
 		if let Some((find_route_query, find_route_res)) = self.next_routes.lock().unwrap().pop_front() {
 			assert_eq!(find_route_query, *params);
 			if let Ok(ref route) = find_route_res {
@@ -201,10 +202,18 @@ impl<'a> Router for TestRouter<'a> {
 					}
 				}
 			}
-			return find_route_res;
-		}
+			route_res = find_route_res;
+		} else {
+			route_res = self.router.find_route(payer, params, first_hops, inflight_htlcs);
+		};
 
-		self.router.find_route(payer, params, first_hops, inflight_htlcs)
+		if let Ok(route) = &route_res {
+			// Previously, `Route`s failed to round-trip through serialization due to a write/read
+			// mismatch. Thus, hwere we test all test-generated routes round-trip:
+			let ser = route.encode();
+			assert_eq!(Route::read(&mut &ser[..]).unwrap(), *route);
+		}
+		route_res
 	}
 
 	fn create_blinded_payment_paths<