que_thr_t::magic_n: Remove. Access to freed data is best caught
by AddressSanitizer.

que_thr_t::start_running(): Replaces que_thr_move_to_run_state_for_mysql()
and que_thr_move_to_run_state(), which were identical non-inline functions.

que_thr_t::stop_no_error(): Replaces que_thr_stop_for_mysql_no_error().

que_fork_t::n_active_thrs, trx_lock_t::n_active_thrs: Make debug-only.

que_fork_t::set_active(bool active): Update n_active_thrs.