set last modified and cache control for varnish

lib/cartodb/cartodb_windshaft.js

@@ -9,6 +9,8 @@ var CartodbWindshaft = function(serverOptions) {
     // set the cache chanel info to invalidate the cache on the frontend server
     serverOptions.afterTileRender = function(req, res, tile, headers, callback) {
         res.header('X-Cache-Channel', req.params.dbname);
+        res.header('Last-Modified','Wed, 30 Aug 1978 00:00:00 GMT'); // special date in the past. 
+        res.header('Cache-Control','no-cache,must-revalidate, public');
         callback(null, tile, headers);
     };