mpm_event: ap_queue_info_try_get_idler() may atomically decrement and then

re-increment the number idlers if it went under or to zero.  We can avoid
this by switching to a compare-and-swap scheme.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1802535 13f79535-47bb-0310-9956-ffa450edef68