core/authority-discovery: Adjust interval to be proactive on

Kademlia's default time-to-live for Dht records is 36h, republishing
records every 24h. Given that a node  could restart at any point in
time, one can not depend on the republishing process, thus starting to
publish  own external addresses should happen on an interval < 36h.

In addition have the first tick of the interval be at the beginning not
after an interval duration.