Move the container start/exit events to the upstream pub/sub event
stream. As these are more of a global event now that we support multiple
commands in the same container, it makes sense to move them to the more
general event stream.
This patch also remove the code in cmd waiting for container start or
exit, making (temporarily) the cmd unable to report power metrics. We
will fix that in a later commit.
This patch fixes complicated issues we had with how a job running
multiple commands in the container might not all wait for the end of the
container: now none of them do.