Add OutgoingPacketsResentCount clientstack stat.

This allows one to monitor the total number of messages resent to clients over time.
A constantly increasing stat may indicate a general server network or overloading issue if a fairly high proportion of packets sent
A smaller constantly increasing stat may indicate a problem with a particular client-server connection, would need to check "show queues" in this case.

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

@@ -145,6 +145,19 @@ namespace OpenSim.Region.ClientStack.LindenUDP
                     stat => stat.Value = m_udpServer.UdpSends,
                     StatVerbosity.Debug));
 
+            StatsManager.RegisterStat(
+                new Stat(
+                    "OutgoingPacketsResentCount",
+                    "Number of packets resent because a client did not acknowledge receipt",
+                    "",
+                    "",
+                    "clientstack",
+                    scene.Name,
+                    StatType.Pull,
+                    MeasuresOfInterest.AverageChangeOverTime,
+                    stat => stat.Value = m_udpServer.PacketsResentCount,
+                    StatVerbosity.Debug));
+
             StatsManager.RegisterStat(
                 new Stat(
                     "AverageUDPProcessTime",
@@ -294,6 +307,16 @@ namespace OpenSim.Region.ClientStack.LindenUDP
 
         public Socket Server { get { return null; } }
 
+        /// <summary>
+        /// Record how many packets have been resent
+        /// </summary>
+        internal int PacketsResentCount { get; set; }
+
+        /// <summary>
+        /// Record how many packets have been sent
+        /// </summary>
+        internal int PacketsSentCount { get; set; }
+
         /// <summary>
         /// Record how many inbound packets could not be recognized as LLUDP packets.
         /// </summary>
@@ -1221,6 +1244,10 @@ namespace OpenSim.Region.ClientStack.LindenUDP
             // Stats tracking
             Interlocked.Increment(ref udpClient.PacketsSent);
 
+            // We're not going to worry about interlock yet since its not currently critical that this total count
+            // is 100% correct
+            PacketsSentCount++;
+
             // Put the UDP payload on the wire
             AsyncBeginSend(buffer);