Previously, we would stop the repeat timer even if we got a null result.
This caused the pending lookup to:
- Never resolve, and
- Never get purged for too many retries
I believe the underlying issue is something on the socket level, but we
should handle this case regardless.
...and make sure it will eventually complete (or fail) by adding a
timeout retry sequence.
Fixes an issue where RequestServer would stick around after exit,
waiting for piled up DNS requests for a long time.
