mpm_worker: replace apr_atomic_cas32(+1) loop with the more efficient

apr_atomic_inc32().
Also declare fd_queue_info_t's idlers member as volatile since it is
used outside atomic functions (or it could be optimized out by the
compiler).


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1643282 13f79535-47bb-0310-9956-ffa450edef68