GDBus: Unref worker from worker-thread to avoid race

GDBus: Unref worker from worker-thread to avoid race

... otherwise we might end up using the worker after it has been
freed. Reported by Dan Winship and Colin Walters.

This fix uncovered a bug in the /gdbus/nonce-tcp test case so "fix"
that as well to use a better way of having one thread wait for another
(using quotes for the word "fix" since it's pretty hackish to
busy-wait in one thread to wait for another).

Signed-off-by: David Zeuthen <davidz@redhat.com>