node: remove asyncStack

Now that the context stores the active execution stack, and because
removeAsyncListener() always removed the AsyncListener from the queue
and the stack, there's no need to keep a stack around anymore. Instead
the active asyncQueue and the currentContext is able to handle it all.


This currently causes two tests to fail, but that were failing on Windows already. Not exactly exactly sure what is going on, but an additional `AsyncListenerInst` is being created for each net connection and firing the `error` callback twice. Investigating why this is.