Mapnik: Bugfix, really reduce list of returned columns from db query

- should have been part of commit 04cb0cc0f423a18c8189e6deeac1965d7a60ad15
Mapnik: group properties into single db column

- properties can be grouped into a single database column; therefore
  only a single string comparison is necessary. As simple tests showed,
  this is way faster (in the OpenStreetBrowser-basemap style for
  Mapnik-2.2 this reduced total rendering time by ~66%).