mpm_event, mpm_worker: Fix cases where the spawn rate wasn't reduced
after child process resource shortages.

The broken scenario:

  child X exits with APEXIT_CHILDSICK
  another child Y is created and reuses child X's scoreboard slot
  child X's exit status is processed

The case that worked is when child X's scoreboard slot hadn't
been reused by the time that its exit status was processed.

Submitted by: trawick
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1364609 13f79535-47bb-0310-9956-ffa450edef68