Prioritize highway classification when joining

[adamfranco]

In Copenhagen I encountered a situation where a tertiary road happened to be split at the junction between it and a service road with the same name. Because of the current Join order, joining diverged from the tertiary road onto the service road, then got stuck there.

When sorting ways prior to joining, order them in descending order of highway class so that we'll be more likely to continue on higher-class ways than diverge onto similarly-named lower-classification ways.