Refactor IScriptInstance / IScriptEngine to use a generic IScriptWorkItem,

which is a thin wrapper around a IWorkItemResult from the SmartThreadPool.
However, it is very easy to reimplement on top of basic threading and therefore
makes the IScriptInstance class independent of the specific threading
implementation.

OpenSim/Region/ScriptEngine/Interfaces/IScriptInstance.cs

@@ -43,6 +43,13 @@ namespace OpenSim.Region.ScriptEngine.Interfaces
         AttachmentCrossing = 2
     }
 
+    public interface IScriptWorkItem
+    {
+        bool Cancel();
+        void Abort();
+        bool Wait(TimeSpan t);
+    }
+
     public interface IScriptInstance
     {
         bool Running { get; set; }