fix child process aborts with

  assertion "!((queue)->nelts == (queue)->bounds)" failed
due to worker queue overflows. The listener/event thread
needs to block when all worker threads are busy.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@109631 13f79535-47bb-0310-9956-ffa450edef68