Adds Surrogate Keys to named maps

lib/cartodb/cache/model/named_maps_entry.js

@@ -0,0 +1,18 @@
+var crypto = require('crypto');
+
+function NamedMaps(owner, name) {
+    this.namespace = 'n';
+    this.owner = owner;
+    this.name = name;
+}
+
+module.exports = NamedMaps;
+
+
+NamedMaps.prototype.key = function() {
+    return this.namespace + ':' + shortHashKey(this.owner + ':' + this.name);
+};
+
+function shortHashKey(target) {
+    return crypto.createHash('sha256').update(target).digest('base64').substring(0,6);
+}