UserTask metrics script should only recalculate durations for new userTask data

Context:

Currently, the userTask metrics scripts iterate through all userTasks currently present on the process instance and recalculate the respective idle and work durations every time new userTask data is imported. This results in an unnecessary amount of calculations and can also lead to race conditions as observed while implementing OPT-4462.

Furthermore, sometimes there appears to be a slight difference between the endTime of a userTask and the endTime of the activity for this userTask which can lead to slight inaccuracies. We should evaluate from a user perspective which durations should "win" during import and, if required, adjust the importers accordingly (currently, OPT-4462 has adjusted the completedActivityImporter to not overwrite start or endtime for new userTask activities.). 
---> after discussing with Felix, Optimize should use the userTask times from task table (ie keep as is). 

We should also consider if it makes sense to move the metricsScript helper to DurationScriptUtil.

AT:

• The userTask metrics scripts only recalcuate durations for newly imported userTask data
• A decision is made as to which end/start time should "win" when importing and, if required, adjustments need to be made (to importers and duration scripts as they would also need recalculating after activity import if the activity start/endtimes should win)