Backported in r1824866.

PATCHES ACCEPTED TO BACKPORT FROM TRUNK:

  [ start all new proposals below, under PATCHES PROPOSED. ]

  *) mpm_queue: Put fdqueue code in common for MPMs event and worker.

     trunk patch: http://svn.apache.org/r1821624

                  http://svn.apache.org/r1821625

                  http://svn.apache.org/r1821626

                  http://svn.apache.org/r1821627

                  http://svn.apache.org/r1821629

                  http://svn.apache.org/r1821632

                  http://svn.apache.org/r1821635

                  http://svn.apache.org/r1821639

                  http://svn.apache.org/r1821644

                  http://svn.apache.org/r1821647

                  http://svn.apache.org/r1821648

                  http://svn.apache.org/r1821649

                  http://svn.apache.org/r1821650

                  http://svn.apache.org/r1821651

                  http://svn.apache.org/r1821659

                  http://svn.apache.org/r1821660

                  http://svn.apache.org/r1822366

                  http://svn.apache.org/r1822367

                  http://svn.apache.org/r1824381

     2.4.x patch: svn merge ^/httpd/httpd/branches/2.4.x-mpm_fdqueue .

                  (http://home.apache.org/~ylavic/patches/httpd-2.4.x-mpm_fdqueue.patch)

     +1: ylavic, minfrin, jim

     ylavic: The branch merge helps resolve move conflicts since event and

             worker fdqueue.[ch] differ(ed) between 2.4.x and trunk. The patch

             is provided only to help review.

     ylavic: re MMN, the API is private, so no bump (supposedly).

     minfrin: Keeping mpm_fdqueue.h private makes it hard for people to fork event

              and worker and make their own mpms. Can we make this public?

     ylavic: not easier/harder than now I suppose (worker/event fqueues are private).

             maybe make them public in trunk only, so that we have more time to

             discuss the API (if needed).

     minfrin: happy with that, we can backport the final API.

  *) mpm_event: Do lingering close in worker(s).

     trunk patch: http://svn.apache.org/r1823047

                  http://svn.apache.org/r1824454