Fix deadlock in agent representation

Deadlock when try to Schedule task "foo", and receive two updates one after another from the agent:
<- update {"foo": stateRunning}
<- update {"foo": stateFailed}

Block when try to signal "outstanding" containers.

Fix: do not stop listening on channel till it's not closed.

@peterbourgon @jmyounker @stuartnelson3
It makes a little more sense to me if this flag is called `sent`, because it's actually flipped to true when we *send* a signal on `successc` or `failurec`. Do you agree?