Check for IP address change every minute, not 15 minutes

Check for IP address change every minute, not 15 minutes

Relays used to check every 10 to 60 seconds, as an accidental side effect
of calling directory_fetches_from_authorities() when considering doing
a directory fetch. The fix for bug 1992 removes that side effect. At the
same time, bridge relays never had the side effect, leading to confused
bridge operators who tried crazy tricks to get their bridges to notice
IP address changes (see ticket 1913).

The new behavior is to reinstate an every-60-seconds check for both
public relays and bridge relays, now that the side effect is gone.