Put a single CalcJob with a transport request

[unkcpz]

This maybe a good solution for #6595 to keep the SSH open for the lifetime of a single CalcJob.

[GeigerJ2]

Thanks for the great discussions on this topic last week, @unkcpz, as well as for looking into it and making the PR. I'm not sure if this is really an ideal solution, though, as it would keep the transport context manager open for the full duration of the CalcJob, which might be hours or days while the job is running and task_monitor_job is being regularly executed.
Wondering what @giovannipizzi thinks about this. For now, it might be easiest to just decrease the DEFAULT_SAFE_OPEN_INTERVAL and call it a day.