traffic: allow RES to buy goods from IND if no affordable COM is around
traffic: save prices when upgrading/downgrading zones

Note: traffic routes are still lost
traffic: reduce performance-impact of enhanced traffic

Here are some measurements made on a small city with population 26,900

CLASSIC performance       :  0.40 millisec per tick

TRAFFIC before this patch : 19.0 millisec per tick

TRAFFIC with this patch   :  5.5 millisec per tick
traffic: use hashmaps to reduce CPU usage tracking traffic

improves performance on same test city as previous commit
to 3.6 millisec per tick.
traffic: raise thresholds for traffic levels