New maximum age policy for notebook servers

[TomAugspurger]

Hi all,

Just wanted to note that we just deployed an update to the Hub that sets a maximum lifetime on the notebook servers of 24 hours.

Previously, we only culled servers that were idle, but "idle" is a bit vague. If you lose your jupyterlab tab in a sea of browser tabs (which happens to me often) it'd be counted as active. These idle but not culled servers were making it hard for users to get spots on the hub, especially for the GPU nodes which are in short supply.

For longer-running jobs, we recommend kbatch, which is designed precisely for the type of long-running batch jobs that you might have been running through the Hub previously, which won't be possible with this new policy.

Let us know if you run into any issues.