count events with arbitrary names, over time, aggregates minutes/hours/days
- counts events with arbitrary names
- stores events per minute (averaging multiple measurements)
- aggregates events per hour/day
open(filename) opens the CountDB readonly
open_for_counting(filename) constitutes a single measurement, stored in a CountDB, allows multiple calls to count(key)
open_for_extending(filename) extends the current CountDB with another one (extend(), used for the aggregation)