wsgi: Start workers in parallel, rather than serially
wsgi: Start workers in parallel, rather than serially

A single worker can take a second or two to start up, which really adds
up when you've got, say, 48 workers.

Change-Id: Id9e8b7e670a67233ac8b79da0b7b8387e302ac02
Related-Change: I83cdaa2cbd394cbd49609c65bf9c5ed026c55417

openstack/swift,openstack/swift,openstack/swift,openstack/swift