Make the late postfork thread a class attribute, and explicitly wait
for it in the test. Fix test failure where the postfork thread was
executed after the test finished.
We do need to include some kind of worker id in the server_name. If we
don't do this and we start with multiple workers the internal
QueueWorker won't work and multiple processes can pick up the same job
if the web process is also a job handler.
Nicola Soranzo