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.

OpenSim/Framework/Monitoring/Stats/Stat.cs

@@ -34,7 +34,7 @@ namespace OpenSim.Framework.Monitoring
     /// <summary>
     /// Holds individual statistic details
     /// </summary>
-    public class Stat
+    public class Stat : IDisposable
     {
         /// <summary>
         /// Category of this stat (e.g. cache, scene, etc).
@@ -181,6 +181,12 @@ namespace OpenSim.Framework.Monitoring
             Verbosity = verbosity;
         }
 
+        // IDisposable.Dispose()
+        public virtual void Dispose()
+        {
+            return;
+        }
+
         /// <summary>
         /// Record a value in the sample set.
         /// </summary>