udevd: make sure a worker finishes event handling before exiting

udevd: make sure a worker finishes event handling before exiting

Persistent network rules write out new rules files. When rules change,
we need to kill all workers to update the in-memory copy of the rules.
We need to make sure, that a worker finshes its work for all device
messages it has accepted, before it exits after a SIGTERM from the main
process.