Don't drop connections during graceful restart. Previously, worker

Don't drop connections during graceful restart.  Previously, worker
threads could exit even though there were connections waiting in the
queue.

Now, for a graceful restart the worker threads won't exit until they
are told that the queue has been drained and no more connections will
ever be added.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94106 13f79535-47bb-0310-9956-ffa450edef68