StepExecution should not contain item-oriented stats #5036
Description
StepExecution currently contains item-oriented stats (read count, write count, etc) even though it does not necessarily represent an item-oriented task.
This is related to #4687, and I guess this came from the idea to have a single abstraction for both single tasks and item-oriented tasks. I believe StepExecution should contain only the basic information about any generic task (start time, end time, etc), while item-oriented stats should live in a subclass like ChunkOrientedStepExecution. Similar thing for a partitioned step where partition info would live in PartitionedStepExecution, and so on.