datacollector: AccumulatedNetworkUsage CollectedData has wrong timestamp

A new AccumulatedNetworkUsage-Object with the current timestamp is generated with each datacollector iteration although they are not fetched always (-> wrong timestamp). Fix needed ASAP!

@usmsk_luebbe Before merging please check if plotting the rates works: No plateaus and no lags (-> should be fixed with Resolve "Plot should be updated with every resp... (!243 - merged))