Fix the enqueue_pool problems :
1. Change working pool from array to hash. When we fire callback & try to remove enqueue obj from working pool, sometimes pool.index didn't match any object, and object will just keep in pool & eat memory.
   So I changed it to hash, within a timestamp. when we do "release_enqueue_pool", we will also clean the objects that remain in enqueue pool which longer than 15 (900) mins. (change it if you need.)
2. fix the trigger of release_qnqueue_pool.