8173658: JvmtiExport::post_class_unload() is broken for non-JavaThread initiators

Call extension ClassUnload event as a deferred event from the ServiceThread and remove unsafe arguments

Reviewed-by: sspitsyn, dholmes

src/hotspot/share/runtime/thread.cpp

@@ -1649,7 +1649,7 @@ void JavaThread::initialize() {
   set_deopt_compiled_method(NULL);
   set_monitor_chunks(NULL);
   _on_thread_list = false;
-  set_thread_state(_thread_new);
+  _thread_state = _thread_new;
   _terminated = _not_terminated;
   _array_for_gc = NULL;
   _suspend_equivalent = false;