Improve not-a-server behavior of server-only timer callbacks

On #3199 (moved), special says:

It makes me a little sad that events like rotate_onion_key, check_ed_keys, record_bridge_stats will be called once per second on clients to handle the unlikely case where the configuration changes and they're now relevant. That is consistent with previous behavior, though. They have a little more overhead than they used to - at least one time() and event_add() each.

We should clean these up now that timers are refactored.