Start an Executor with specific job to run only

[baminou]

No description provided.

[junjun-zhang]

Normally a job in queued state can be picked up by any running Executor. If we want an executor to pick up a particular job, it may not always succeed since there is an opportunity that another executor may have already picked up the specific job. In order to guarantee a specific job only picked by a specific executor there are two options (maybe more):

1. add a piece of information for preferred executor to a job when the job enters into queued state.
2. put the job into the on-hold state to prevent it from being picked by any executor, then specify the job_id when starting the executor the user wants it pick up the specific job

The second option is more plausible in terms of implementation and on-hold state is something useful on it own when investigation is needed for certain jobs.