Pick render limit and add it to request context

- Extends the problematic fake request in templates
 - Picks the value in waterfall, this must be improved because:
   1. It does not make sense if there is no layers with limits
   2. If we want to include it always without considering the layer type
      we can do the operation in parallel

lib/cartodb/controllers/template_maps.js

@@ -408,7 +408,8 @@ TemplateMapsController.prototype.instantiateTemplate = function(req, res, templa
             if ( req.profiler ) req.profiler.done('TemplateMaps_instance');
             if ( err ) throw err;
             layergroup = instance;
-            fakereq = { query: {}, params: {}, headers: _.clone(req.headers),
+            fakereq = {query: {}, params: {}, headers: _.clone(req.headers),
+                context: _.clone(req.context),
                 method: req.method,
                 res: res,
                 profiler: req.profiler