Provide OverviewsHandler configuration to Windshaft

A parameter has been added to Windshaft Mapnik renderer configuration
to define how queries will be adapted to use overviews.

Here we're using the default OverviewHandler providen in Windshaft,
with a parameter to define how the zoom level is determined.

lib/cartodb/server_options.js

@@ -1,5 +1,11 @@
 var os = require('os');
 var _ = require('underscore');
+var windshaft = require('windshaft');
+var OverviewsHandler = windshaft.OverviewsHandler;
+
+var overviewsHandler = new OverviewsHandler({
+  zoom_level: 'CDB_ZoomFromScale(!scale_denominator!)'
+});
 
 var rendererConfig = _.defaults(global.environment.renderer || {}, {
     cache_ttl: 60000, // milliseconds
@@ -10,11 +16,13 @@ var rendererConfig = _.defaults(global.environment.renderer || {}, {
         bufferSize: 64,
         snapToGrid: false,
         clipByBox2d: false,
-        limits: {}
+        limits: {},
     },
     http: {}
 });
 
+rendererConfig.mapnik.overviewsHandler = overviewsHandler;
+
 // Perform keyword substitution in statsd
 // See https://github.com/CartoDB/Windshaft-cartodb/issues/153
 if ( global.environment.statsd ) {