relay: Look at our cache when looking for an IP change

Regularly, tor looks if its IP has changed. It does the entire auto discovery
process again. However, it is possible that it does not find anything.

Instead of thinking the IP changed to an unknown address, look at our cache
and see if that value has changed.

The reason for this is because if tor gets its address as a suggestion from a
directory authority, it is because the auto discovery failed and thus that
address should be consider for the IP change check.

Related to #40071

Signed-off-by: David Goulet <dgoulet@torproject.org>