place=postcode places are artificial places that collect addr:postcode
points for aggration. They should neither show up in the address nor
be searchable. That means that there is no need to index them at all.
Only let boundary=postal_code through which define correct areas for
postcodes.
Many of the postcode nodes are actually derived from
incomplete addresses and are as such not even centroids.
Better let them only take part in the address computation
via the postcode table.
When roads cross boundaries, both administrative entities should
be added to the address list, so that both entities can be used
for searching. Also allows in a second step to better sort out
addresses of POIs on such roads.
Fixes#121.
