Prevent container users from causing a load-peak at a certain fixed
schedule. There are lots(^1) of reports that the official "every even
five minutes"-update schedule is unreliable and a delay helps.
It stands to reason that enough users with any one schedule can cause
the same problem.
It's also much nicer for duckdns' servers, if queries arrive
uniformly distributed instead of at certain peaks.
As a practical consideration for the known problem at or near even five
minutes, completely avoid timings 10s before them and 60s after them.
(^1) https://github.com/SensorsIot/IOTstack/issues/271
