Before this, the notification semaphore was broken() in do_abort(),
which was triggered by early abort source.
However we are going to reload sl cache on topology state reload
and it can happen after the early abort source is triggered, so
it may throw broken_semaphore exception.
We can move semaphore breaking to stop() method. Legacy update loop
is still stopped in do_abort(), so it doesn't change the order of
service level controller shutdown.
cf29242962