mpm_event: Retain active_daemons instead of resetting it on restart.

mpm_event: Retain active_daemons instead of resetting it on restart.

Since active_daemons is tracked by perform_idle_server_maintenance() and
decremented when a child gets quiescing or exits, clearing it on restart
is actually an off by -total_daemons (of the old gen) after the first calls
to perform_idle_server_maintenance().

Let perform_idle_server_maintenance() be the only one to update active_daemons
by putting it in the retained struct to keep track accross restarts.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1895550 13f79535-47bb-0310-9956-ffa450edef68