Adds Surrogate Keys to named maps

lib/cartodb/cache/surrogate_keys_cache.js

@@ -0,0 +1,26 @@
+/**
+ * @param cacheBackend should respond to `invalidate(cacheObject, callback)` method
+ * @constructor
+ */
+function SurrogateKeysCache(cacheBackend) {
+    this.cacheBackend = cacheBackend;
+}
+
+module.exports = SurrogateKeysCache;
+
+
+/**
+ * @param response should respond to `header(key, value)` method
+ * @param cacheObject should respond to `key() -> String` method
+ */
+SurrogateKeysCache.prototype.tag = function(response, cacheObject) {
+    response.header('Surrogate-Key', cacheObject.key());
+};
+
+/**
+ * @param cacheObject should respond to `key() -> String` method
+ * @param {Function} callback
+ */
+SurrogateKeysCache.prototype.invalidate = function(cacheObject, callback) {
+    this.cacheBackend.invalidate(cacheObject, callback);
+};