Set ThreadPriority on main scene thread to highest.

This will only have an effect on Windows systems or mono with the (not recommended) mono-3.2.8 debug patch https://gist.github.com/justincc/31e52218d098529b4696 applied

OpenSim/Region/Framework/Scenes/Scene.cs

@@ -1451,6 +1451,7 @@ namespace OpenSim.Region.Framework.Scenes
             }
             else
             {
+                Thread.CurrentThread.Priority = ThreadPriority.Highest;
                 Update(-1);
                 Watchdog.RemoveThread();
                 m_isRunning = false;