Do not hold mutex while calling que_graph_free()

Ever since InnoDB table cache eviction was implemented, it became necessary to invoke dict_table_close() to release table handles. Because this call requires the dict_sys->mutex to be held, the mutex currently needs to be held also while invoking que_graph_free(). This feels very inefficient and cumbersome. Could we streamline dict_table_close() further, so that no mutex is needed in this case? Note: dict_table_t::release() already uses atomic memory operations.