Support custom restart step/position

For a failed job execution, the subsequent restart will automatically restart from where it failed.

For a stopped job execution, the subsequent restart will restart from where it was stopped, or the step specified by the <stop restart="xxx"> attribute.

If any steps in the job.xml has allow-start-if-complete attribute set to true, those steps will also be run regardless if they already completed or not in the prior job execution.

However, sometimes it's desirable to restart from a restart position different from the above specification, for reasons like:

• <stop> element has restart attribute, but <fail> element does not, and users want to do the same for a failed job execution.
• <stop> element in job.xml does not specify restart attribute (maybe forgot).
• after the job execution had failed or stopped, things have changed, or due to manul intervention, the default restart position should not be used.