another postgres provider update:

- make declareCursor and getFeature private
- there's no need to track column names in cursors
  (which contained a bug triggering #962 again and more)
- introduce private method field(index) that verifies that
  the index in valid or throws an exception.
- use that in public methods that get indexes from
  outside (minValue, maxValue, uniqueValues...)


git-svn-id: http://svn.osgeo.org/qgis/trunk/qgis@8290 c8812cc2-4d05-0410-92ff-de0c093fc19c