- Weaken internal core MCE reference to reap workers automatically
upon leaving the { scope i.e. omitting shutdown }. Note: No change
to MCE models Flow, Grep, Loop, Map, Step, and Stream. Call finish
explicitly to reap workers.
This resolves the case where the author omitted calling $mce->run(),
$mce->run(1), or $mce->shutdown() inside a scope, causing workers
to linger around until completion of the script.