reworked synching of session shutdown with worker threads, workers now stick...

reworked synching of session shutdown with worker threads, workers now stick to a session until no more reuqquest are tbd, keepalive handling revisited

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1722899 13f79535-47bb-0310-9956-ffa450edef68