Add polygon simplification

Add polygon simplification

New query string parameter polygon_threshold=<0.0..1> is introduced.
The float value of this parameter (defaults to 0) is passed to
ST_SimplifyPreserveTopology() on geometry we're about to output in
one (or many) requested formats such as GeoJSON, KML, etc.

This is useful when getting border polygons for whole countries, but
rendering them at large scale, when most of the high resolution details
cannot be seen anyway.  For example, the unsimplified polygon data for
Germany in GeoJSON format currently makes for about 3 MB response body.
With use of this new parameter, the application can greatly reduce the
amount of downloaded data and server response time while providing its
users with the same picture.  On a typical laptop screen resolution,
zooming out to fit the whole country borders on screen, only 1/100 amount
of details could be well enough.