more trustworthy downtime tracking
Detection chore: Do not update downtime at all from the detection chore.
We only want to include downtime between two explicitly failed ping attempts
(the duration between last contact success and the first failed ping is no longer
included in downtime calculation)
Estimation chore: If the satellite started after the last failed ping for a node,
do not include offline time since the last failed ping time - only
estimate based on two failed pings with no satellite downtime in
between.
This protects us from including satellite downtime in our storagenode downtime calculations.
Change-Id: I1fddc9f7255a7023e02474255d70c64faae75b8a
