User xyz requested server for qrs, which they don't own

[robnagler]

Related to #19.

1. User qrs tries to get at pre-allocated container, but fails because of removing the container is asynchronous and causes exceptions #19 and remove starts and slot N is freed.
2. User xyz gets allocated slot N and starts differently named container (no conflict) "juptyer-xyz"
3. JupyterLab tries to get a file via proxy, because routes are added right away (before container starts)
4. Proxy forwards to jupyter-qrs container via slot N (port M statically assigned) and gets conflict

[e-carlin]

I can re-create this.

1. Set min_activity_hours to 0.002
2. Set servers_per_host to 1
3. Login as user a
4. Login in a new browser (or icognito window) as user b
5. You should see the error. If not start a's server then try and start b's server do this a couple times and you will see it.

I'm running jupyterhub using container-jupyterhub/container-conf/test.sh.

Still not very clear what causes this issue. I've tried to implement polling in rsdockerspawner.remove_object to make sure the socket is closed but it seems remove_object isn't called.