change default rank for highway objects to 30

change default rank for highway objects to 30

The highway key is being used more and more for non-ways these
days. This clashes with Nominatim's assumption that essentially
everything that has a highway tag can be used as the street part
of the address.

Change the default rank of highway objects to 30 to avoid this.
Only the known values for streets keep the rank 26 and are now
listed explicitly.