Perfmon-Hook-Task-5a Implement Report Overdue Timer

The PerfMon design calls for an interval timer to expire and report MonitoredDurations whose current intervals have expired. Interval completion for a given MonitoredDuration is typically detected as new samples are added to it. Its current interval is completed, reported, and a new current interval begun. When traffic stops, some MonitoredDurations will likely have an incomplete current intervals (i.e. unreported data) that will not report until or unless traffic resumes. The overdue timer is meant to flush these intervals when updates to them have ceased. I have it implemented but it lacks UTs and the added complexity and additional performance overhead may not be needed. I created this issue to cover adding this functionality, should it be deemed useful or wanted by users. The initial implementation (lacking UTs and doxygen) is in !2269.