restart fixes for worker:

fix a segfault and a window in which we could miss joining
newly-created threads

we can't try to signal workers if the worker queue hasn't
been initialized (or we segfault)

make sure the start thread is done creating threads before
we try to join; otherwise we can just miss some of them and
not be able to clean them up properly


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93561 13f79535-47bb-0310-9956-ffa450edef68