mpm_event: Restart stopping of idle children after a load peak. PR 65626.

mpm_event: Restart stopping of idle children after a load peak. PR 65626.

r1770752 added an heuristic to avoid stopping children when the load triggers
MaxSpareThreads but children take some time to shut down until the point where
active_daemons_limit/ServerLimit is reached (scoreboard full) and no child gets
created to handle incoming connections.

However when this happens there is nothing to stop children again when the load
settles down (besides MaxRequestsPerChild, which may be 0) so let's restart to
stop children again if/when idle_thread_count reaches max_workers / 4.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1894285 13f79535-47bb-0310-9956-ffa450edef68