Add a first draft mechanism for the IncomingPacketsProcessedStat to show the delta over time.

The chief motivation for this is to be able to tell whether there's any impact on incoming packet processing from enabling extra packet pooling.

OpenSim/Region/ClientStack/Linden/UDP/LLUDPServer.cs

@@ -80,6 +80,7 @@ namespace OpenSim.Region.ClientStack.LindenUDP
                     "clientstack",
                     scene.Name,
                     StatType.Pull,
+                    MeasuresOfInterest.AverageChangeOverTime,
                     stat => stat.Value = m_udpServer.IncomingPacketsProcessed,
                     StatVerbosity.Debug));
         }