Finish implementation of proper IdentityFetcher shutdown:
- onFound() threads can be pending after stop(): They might have been waiting for the synchronized() block. So don't log a problem if stop() has killed the request which triggered onFound(). But do log an error if we are not stopped because then there isn't ANY reason for the request to exist
- This fixes issue https://bugs.freenetproject.org/view.php?id=5830