dht: uniformly sync SearchNodes reannouncing

Historically, the next announce timing was based on the last acknowledgement
received from a SearchNode. This behavior was invalid since the 'refresh'
operation. In fact, for a given value that was "refreshed", its creation time
would differ from the announcer's info. Therefor, all search node's
reannouncing timings would differ, hence causing inconsistency on the network.