increase notification cooldown

[GRarer]

users who have an above-average number of friends are getting notifications too often. since we don't want to completely rewrite the scheduling algorithm, the solution to this is to increase the cooldown, but we'll probably want to also shorted the interval to counterbalance this.

[GRarer]

I wasn't entirely confident that my original stochastic-simulation code was correct, for for the retuning i've rewritten it: link

between the new code and a lower target for average pushes-per-day, the probabilities will be reduced significantly