geolocation: filter cached dzd measurements

[ben-dz]

geoprobe-agent should do some filtering on DZD cached measurements so that we don't get high bounces.

Keep the lowest dzd reference that's valid, but then keep a backup one that is somewhat newer so that we aren't left without one when the low one expires. Perhaps never replace the lowest one until it expires after the full timeout, but always keep a separate copy of the lowest in the most recent half-timeout interval. That way when a full timeout expires, there's a new decent measurement with at least a half-timeout lifespan left.