Kea statistics enhancements

Kea is currently able to report dozens of statistics. However, for each statistic there is only one specific value being reported. For certain types of activities it is highly desirable to have multiple observations over time. Having many data points gives an insight into processes that are changing over time, e.g. daily patterns in user activities, DOS detection and mitigation etc. Therefore the primary task of that issue is expanding the method of collecting observations in Kea by adding the possibility to collecting statistics as time-series ‘buckets’.