Exit threads early during shutdown, part 2

Follow up to r1738466: During graceful shutdown, when the listener thread is
closing a connection, it needs to wake up a worker thread so that it may
terminate.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1738486 13f79535-47bb-0310-9956-ffa450edef68