- fts_optimize_thread() uses dict_table_t object instead of table id.
So that it doesn't acquire dict_sys->mutex. It leads to remove the
hang of dict_sys->mutex between fts_optimize_thread() and other threads.

- in_queue to indicate whether the table is in fts_optimize_queue. It
is protected by fts_optimize_wq->mutex to avoid any race condition.

- fts_optimize_init() adds the fts table to the fts_optimize_wq