Defer starting the listener until we have at least tried to start all the worker

Defer starting the listener until we have at least tried to start all the worker
threads.  We want to get the workers in the pool of available threads ASAP
to keep perform_idle_server_maintenance from thrashing and starting too
many processes prematurely.  The code before this patch would dribble workers
into the worker pool over an extended period of time and this prevented
perform_idle_server_maintenance from accurately deciding when new processes
were needed.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94889 13f79535-47bb-0310-9956-ffa450edef68