Make outboudn and packet inbox handling threads highest priority.

Will only have any affect under Windows or mono with a patch such as https://gist.github.com/justincc/31e52218d098529b4696 (not recommended) applied.
For assessment purposes.

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

@@ -2183,6 +2183,8 @@ namespace OpenSim.Region.ClientStack.LindenUDP
 
         private void IncomingPacketHandler()
         {
+            Thread.CurrentThread.Priority = ThreadPriority.Highest;
+
             // Set this culture for the thread that incoming packets are received
             // on to en-US to avoid number parsing issues
             Culture.SetCurrentCulture();
@@ -2228,6 +2230,8 @@ namespace OpenSim.Region.ClientStack.LindenUDP
 
         private void OutgoingPacketHandler()
         {
+            Thread.CurrentThread.Priority = ThreadPriority.Highest;
+
             // Set this culture for the thread that outgoing packets are sent
             // on to en-US to avoid number parsing issues
             Culture.SetCurrentCulture();

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

@@ -217,6 +217,8 @@ namespace OpenSim.Region.ClientStack.LindenUDP
 
         private void ProcessRequests()
         {
+            Thread.CurrentThread.Priority = ThreadPriority.Highest;
+
             try
             {
                 while (IsRunning || m_requestQueue.Count > 0)