udevd: worker - drop reference counting

udevd: worker - drop reference counting

Make the worker context have the same life-span as the worker process. It is created on fork()
and free'd on SIGCHLD.

The change means that we can get worker_returned() for a worker context that is no longer around,
this is not a problem and we can just drop the message. The only use for worker_returned() is to
know to reschedule events to workers that are still around, so if the worker has already exited
it is not important to keep track of. We still print a debug statement in this case to be on the
safe side.