Added query possibility in HistoricProcessInstance to search for process instances that have a specific super process instance.
This makes it possible to retrieve sub processes created via a call activity based on the parent process instance id.

After some rethought I stumbled upon unnecessary code in the GetExcecutionVariablesCmd, so now it makes more sense

Changed collectVariables so that local variables override parent variables and added a unit test VariableScopeTest to show how it works