Parallel bulk insert/delete/update based on parallel select
handler->ha_get_bg_result: means wait for the background thread in handler API, maybe we can use it in commit/rollback interface.
add parameter of "sql_type" in spider_bg_conn_search , to distinguish select and the other type.
spider_set_conn_bg_param_for_dml: check whether it can insert parallel mode. Maybe it should be decided by a new spider parameter
spider_db_bulk_insert: main logic for parallel insert_lock_tables_list
spider_bg_conn_action: reuse the most logic. Take careful of dbton_handler->set_sql_for_exec