MINOR: listeners: check the current listener state in pause_listener()

MINOR: listeners: check the current listener state in pause_listener()

It's better not to try to perform pause() actions on wrong states, so
let's check this and make sure that all callers are now safe. This
means that we must not try to pause a listener which is already paused
(e.g. it could possibly fail if the pause operation isn't idempotent at
the socket level), nor should we try it on earlier states.