traverse address list backwards when computing admin levels

traverse address list backwards when computing admin levels

By starting with the lowest address level, when collecting
administrative boundaries by level, there is a better chance
to actually get the boundary when the capital of the
administrative boundary is tagged with the level of the
boundary as well.

This is just a heuristics adaption to make the best out of
the imprecise admin_level definition for place nodes.

Fixes #1261.