Fix refreshing cached A(AAA) records that expire

Old code was trying to refresh record after it has already expired. Now
it tries after 50% of TTL has passed - we do the same for other records.

Fixes: 80dd24602480 ("Refresh DNS records A and AAAA directly")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: John Crispin <john@phrozen.org>