Aggregate privacy budget tracking / management is difficult

[csharrison]

In AGGREGATE.md each histogram contribution consumes some L1 budget locally, which is periodically reset after some time window passes. Managing this budget is difficult, especially when the reset periods are long. API users have no idea how much budget is left, and can only guess at how much to consume at any one time. We should consider techniques to make managing the budget easier.

One simple technique is to add the budget remaining as a parameter into the worklet. This should be privacy neutral and allow the worklet to make more fine-tuned decisions about how much more budget to consume.

We can also consider shrinking the windows for budget resetting, which would make decisions carry less weight as they would affect fewer future events.