[Dextrose] Stability stuff

[Martin Abente]

Weird, I really tried to trigger it on our last Dextrose build and never
happened.

The whole idea of killing activities is a little bit controversial I
think, you have to assume to many things about activities, so far just a
few activities in sugar uses all the proper mechanisms, I am afraid that in
most of the cases kids would just loose their current work.

What about... If the system load is already close to a "critical" point,
SUGAR could just stop new activities from being executed with a proper
warning, and suggestions.