* We use ThreadLocal to trans SceneScriptManager context to ScriptLib, to avoid eval script for every groups' trigger in every scene instances.
* But when callEvent is called in a ScriptLib func, it may cause NPE because the inner call cleans the ThreadLocal so that outer call could not get it.
* e.g. CallEvent -> set -> ScriptLib.xxx -> CallEvent -> set -> remove -> NPE -> (remove)
* So we use thread pool to clean the stack to avoid this new issue.