Shared cache for affected tables in layergroup and map controllers

lib/cartodb/cache/model/database_tables_entry.js

@@ -15,6 +15,10 @@ DatabaseTables.prototype.key = function() {
     }.bind(this));
 };
 
+DatabaseTables.prototype.getCacheChannel = function() {
+    return this.dbName + ':' + this.tableNames.join(',');
+};
+
 function shortHashKey(target) {
     return crypto.createHash('sha256').update(target).digest('base64').substring(0,6);
 }