docker watcher proposal: use the events API instead of polling

[dnephin]

Replace polling by streaming the events API

• using the events API filter by the start event to get notified of all new containers that have started
• when a new container starts, check it against the list of criteria and register it if it matches
• removes the need to poll, and keep track of the list of previous containers
• the /containers/json endpoint that it uses now can be pretty slow with large numbers of containers

[dnephin]

cc @bobtfish, @bcicen, @rgarcia

You've all worked with this watcher or have pending PRs for it. What do you think about this idea?

[bobtfish]

👍