idle jobs must self destruct

[bjuergens]

Problem: the jobs will never finish. REQUESTED_WALLTIME may or may not work

Ideas:

• REQUESTED_WALLTIME does not know if the server is idle.
• condor_ssh_to_job dies when the
• Running Notebooks/Terminals - Shutdown automatically after specified time jupyter/notebook#1300 (comment)

possible solution:

• set REQUESTED_WALLTIME to 8h (enough for the user to get all the work done)
• set NotebookApp.shutdown_no_activity_timeout = 600
• set MappingKernelManager.cull_idle_timeout = 600

result:

• the server shuts down 20 minutes after the last kernel went idle
• when there is still an open ssh connection at this point, the connection stays open, but won't
• if the user forgets to log out, the job is culled after 8h