Review https://github.com/jupyterhub/jupyterhub-idle-culler/pull/83

[yuvipanda]

jupyterhub/jupyterhub-idle-culler#83 is a nice little PR that is a good refactor, which will enable the idle culler to be more customizable in the future. It doesn't add any real new features, and is mostly pure refactor. Has been sitting around since november, and will be good to review and merge. This goes into z2jh eventually!

rcthomas is a pretty good and responsive contributor!

Definition of done

• Replace tornado options with traitlets jupyterhub/jupyterhub-idle-culler#83 is reviewed (and ideally merged)
• Experience reported in Round 1: Strategic upstream OSS contributions #5058