Add CounterStat which is a wrapper for a counter stat but, because
the 'count' event is internal, historical data can be built it.
Also includes EventHistogram class for building time based,
bucketed history of event occurances.
Make Stat implement IDisposable for subclasses that might need it.
Add a virtual ToOSDMap() function to Monitoring.Stat for future
overloading by aggregation statistics and browser based stat viewers.