reduce API requests by monitoring state using `docker.events`

[minrk]

trafficstars

We should follow kubespawner's example and monitor container events instead of querying the docker API every time.

Most importantly, if we can avoid .poll() making any API requests, instead referring directly to the event stream, that should improve the scalability quite a bit.

[yuvipanda]

Yeah, scalability improved massively for kubespawner with this change. +1 for making it happen for docker too!