Make the polling period constant

fix #173

The polling is done in a strange way… We used to keep a list of stuff to be polled, ordered by the time they should be polled.

And once in a while this list was "tuned", which is more or less reordered and balanced, but in the process the time for next polling is changed, thus causing the issue we've seen

So, I switched to a multiset instead of a list to have ordering for free, and got rid of the "tuning" step, so the polling time are not changed anymore. The balancing step is out as well, but I'm skeptical it was useful in the first place…