Consider updating overload-general timestamps

In order to determine if relays have recently or frequently experienced overload, we could patch check_descriptor_callback() to republish the descriptor if the overload-general timestamp was recently updated.

We would also need to patch router_differences_are_cosmetic(), so that dirauths decide to include this descriptor.

The upsides of doing this are that it lets us know if overload is recently and continuously happening, so that analysis like tpo/network-health/team#66 (comment 2773945) and sbws usage becomes easier.

The downsides of frequent update are giving an adversary an immediate feedback that a DoS attack is effective.

Cc: @dgoulet, @gk