Cleanup name handling in the Thread class and subclasses

We can simplify and cleanup up name() management

- make name() return "const char *" and only cast away constness at API boundaries when essential
- add type_name() so that we can avoid code like "if (t->is_VM_Thread()) print("VMThread");
- Rename JavaThread::get_thread_name() to name() (no need for the extra indirection)
- avoid need for special handling of NamedThread