tpool - switch off maintenance timer when not needed

Reported by monty:
The maintenance timer is there to prevent deadlocks, i.e create additional threads when tpool does not make progress. It is only rarely needed, but it still wakes up every 400 milliseconds.
It could be switched off, or frequency lowered, when server is idle.

Update April 17 2024. It is not possible to switch that timer entirely OFF, attempts to do this are deadlock prone (mostly due to mysys thr_timer, which holds global lock for entire duration of the callback). If we ever try out luck, then with another timer implementation, not mysys.
However switch between longer and shorter timer periods are still possible, so this path will be taken