Change rendering country/state labels

* Country and state labels are now rendered from the admin borders in the polygons
  table rather than from the place nodes in the points table.
* The maximum zoom level requirement for country/state labels is replaced by a
  maximum area size requirement.
* A minimum area size requirement for country/state labels is added.

Advantages:
* Avoid cluttering of too many labels of small areas, in particular on z4 and z5.
* Easier to detect broken country and state multipolygons.

Disadvantage:
* This requires that country and state multipolygons are not broken, otherwise
  the labels won't render.