Improve named map config format for cache key

lib/cartodb/models/mapconfig/named_map_provider.js

@@ -1,5 +1,6 @@
 var _ = require('underscore');
 var assert = require('assert');
+var crypto = require('crypto');
 var dot = require('dot');
 var step = require('step');
 var MapConfig = require('windshaft').model.MapConfig;
@@ -155,13 +156,20 @@ NamedMapMapConfigProvider.prototype.createKey = function(base) {
         owner: this.owner,
         templateName: this.templateName,
         authToken: this.authToken || '',
-        configHash: this.config && this.config.toString() || '',
+        configHash: configHash(this.config),
         layer: '',
         scale_factor: 1
     });
     return (base) ? baseKeyTpl(tplValues) : rendererKeyTpl(tplValues);
 };
 
+function configHash(config) {
+    if (!config) {
+        return '';
+    }
+    return crypto.createHash('md5').update(JSON.stringify(config)).digest('hex').substring(0,8);
+}
+
 NamedMapMapConfigProvider.prototype.setDBParams = function(cdbuser, params, callback) {
     var self = this;
     step(