Delay calculator adds the delta on each call instead of update it from TH time

Error detected when DelayCalculationDataProcessingWorker::_processVpDelay is called:

If bus is 2 minutes late, at each call it will add that time to the total retard: 2, 4, 6 minutes etc. So if the job runs every 5 seconds, after 1 minute it will pretend bus is 24 minutes !! (instead of 2 all along)

See around VehiclePosition::computeDelay() and PTRealTimeUpdate