Support different forward/backward ref values on edges

[danpat]

We're adding support for cardinal directions on route relations for North American road networks.

However, single carriageway (non-divided) roads that belong to routes with cardinal directions require the ability to store different values depending on the direction of travel.

A simple example is this way: http://www.openstreetmap.org/way/490434780

This belongs to CA 74 West in one direction, and CA 74 East in the other. The way itself has a ref of CA 74.

There are more complex examples as well, where roads have completely different refs in each direction (ref:forward/ref:backward).

We should support edges that have different ref values in each direction - this would give us complete support for cardinal direction information in North America.