Consider renaming ClassLoaderData::keep_alive

When iterating over the ClassLoaderDataGraph and handing out CLDs, you sometimes need to make sure that the CLD and its oops are kept alive. I think it would be easy to think that you should call ClassLoaderData::keep_alive() to do that, just like we have CollectedHeap::keep_alive(oop). However, ClassLoaderData::keep_alive already exists and returns whether the CLD is forcefully kept alive or not. So, what you need to do today is to call ClassLoaderData::holder(), and that will keep the CLD alive.

I propose that we rename the current ClassLoaderData::keep_alive() to something else, and then use ClassLoaderData to apply the "keep alive" property.