XD-1644 Deduce JobExecution restart status

- For the given JobExecution, set the restart status by verifying
    all the job executions of the same `JobInstance` and set the
    job execution to be `restartable` only if none of the job executions
    are in `BatchStatus.COMPLETED` status.
- Update this logic only in listJobExecutions() in BatchJobExecutionsController
    so that the origin JobExecutionInfoResource is not affected in other cases.
- Fix JobCommandTests to identify the JobExecution that is either `STARTING` or `STARTED`
- Refactored `restartable` flag setting logic at BatchJobExecutionsController