terminals excluded from activity monitoring

[minrk]

Kernels are monitored for activity and can be shutdown when they have been idle for some period of time. Terminals, on the other hand, are not observed for activity and therefore never treated as idle. This has two consequences:

1. terminals cannot be shutdown by some idle timeout
2. if any terminal is started and not terminated, shutdown_no_activity will never teardown the server itself.

This would help e.g. jupyterhub deployments that want to shutdown after some idle time.

[kevin-bates]

I think we need to port jupyter/notebook#5372 to server (and make jupyter/notebook#5465 is there as well).

[minrk]

Awesome! I didn't notice this had landed in the classic server already.

[kevin-bates]

We also need to bring jupyter/notebook#5462 to the party as well.