MINOR: listeners: count unstoppable jobs on creation, not deletion

MINOR: listeners: count unstoppable jobs on creation, not deletion

We have to count unstoppable jobs which correspond to worker sockpairs, in
order to know when to count. However the way it's currently done is quite
awkward because these are counted when stopping making the stop mechanism
non-idempotent. This is definitely something we want to fix before stopping
by protocol or our listeners count will quickly go wrong. Now they are
counted when the listeners are created.