routing: add proportional attempt cost

sample-lnd.conf

@@ -619,8 +619,12 @@ litecoin.node=ltcd
 ; probability (default: 1h0m0s)
 ; routerrpc.penaltyhalflife=2h                            
 
-; The (virtual) cost in sats of a failed payment attempt (default: 100)
-; routerrpc.attemptcost=90                                
+; The (virtual) fixed cost in sats of a failed payment attempt (default: 100)
+; routerrpc.attemptcost=90     
+
+; The (virtual) proportional cost in ppm of the total amount of a failed payment
+; attempt (default: 1000)
+; routerrpc.attemptcostppm=900
 
 ; The maximum number of payment results that are held on disk by mission control
 ; (default: 1000)