MPMs: early initialize scoreboard's child generation number.

Since [mpm]_note_child_killed uses the scoreboard's generation number for
child_status hook (MPM_CHILD_EXITED), we must initialize it early (i.e. in
[mpm]_note_child_started where MPM_CHILD_STARTED is set) to avoid race
conditions on restart (e.g. storm/loop of restarts) leading to AH00546.

PR 62658.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1839571 13f79535-47bb-0310-9956-ffa450edef68