Spin off NPC deletion into a thread to avoid it being done on a script

microthread. A stab at fixing exceptions.

OpenSim/Region/ScriptEngine/Shared/Api/Implementation/OSSL_Api.cs

@@ -2406,7 +2406,9 @@ namespace OpenSim.Region.ScriptEngine.Shared.Api
                 if (!module.CheckPermissions(npcId, m_host.OwnerID))
                     return;
 
+                Util.FireAndForget(delegate(object x) {
                     module.DeleteNPC(npcId, World);
+                });
             }
         }