stop jhub while rudaux is running

[trevorcampbell]

right now rudaux just stops/starts the jhub when it creates user accounts. But if TAs are working, it might kill their session. Perhaps just take jhub down once at the beginning of the grading flow, and bring it back up at the end. Check if people are working on jhub before bringing down.

then again, new JHub setup with configurable proxy etc is fine if we take jhub down while people are working, so maybe this can just be closed. I'll leave it open for now to document the potential idea anyway.