BATCH-2008: Implementation of JSR-352 JobInstance, JobExecution and StepExecution

* Spring Batch's native JobInstance now implements the JSR's JobInstance interface.
* SB's JobExecution can be wrapped by a JSR's JobExecution.
* SB's StepExecution can be wrappedy by a JSR's StepExecution.
* A new JsrFlowJob and JsrFlowExecutor have been introduced to correctly handle the promotion of ExitStatuses during step execution.
* The JsrJobOperator has been fixed to correctly retrieve StepExecutions (previously was not returning fully hydrated verions).