containerd executor should be smarter about containerd events subscription

[ijc]

The containerd executor added in #1965 currently starts a containerd event stream for every task. These event streams contain events for all containers which containerd is managing. We should either:

• Make use of server side filtering in containerd (not yet implemented) to request a per-task stream which only contains events relevant to the task.
• Setup a singleton event stream on the client side which demuxes into multiple per task event streams.

My preference is to do the former.

[ijc]

With #2275 this issue is pushed down into the container.Task.Wait() method. The code puts @ehazlett on the hook for that, but I'm not sure if there is an issue over there yet.

[ijc]

Thanks! I'll leave this here at least until #2275 is merged I think.

[ijc]

Correct containerd link: containerd/containerd#1066