Best to distinguish CreateTime and StartTime in StepExecution

[lcmarvin]

I need to record create time of StepExecution, but I found it only has a filed startTime, though this field has been assigned value when StepExecution is created.

private volatile Date startTime = new Date(System.currentTimeMillis());

But its value will be updated when StepExecution gets started in AbstractStep.

stepExecution.setStartTime(new Date());
stepExecution.setStatus(BatchStatus.STARTED);

So, maybe an individual createTime field is more suitable, just like what it is in JobExecution.

private volatile Date startTime = null;
private volatile Date createTime = new Date(System.currentTimeMillis());

[lcmarvin]

This requires following changes at least:

1. Schema updates, table bath_step_execution needs a new column create_time.
2. Class StepExecution needs a new filed createTime.
3. SQL in org.springframework.batch.core.repository.dao.JdbcStepExecutionDao#SAVE_STEP_EXECUTION, org.springframework.batch.core.repository.dao.JdbcStepExecutionDao#GET_RAW_STEP_EXECUTIONS, and org.springframework.batch.core.repository.dao.JdbcStepExecutionDao#GET_LAST_STEP_EXECUTION need to be updated to include create_time.

[lcmarvin]

Please refer to PR #4057

[lcmarvin]

Could you please link another pull request #4099 to this issue? Since a wrong rebase was done by mistake in #4057, so I close it.

Sorry for that, Thanks！