MINOR: config: disable busy polling on old processes

MINOR: config: disable busy polling on old processes

in the context of seamless reload and busy polling, older processes will
create unecessary cpu conflicts; we can assume there is no need for busy
polling for old processes which are waiting to be terminated.

This patch is not a bug fix itself but might be a good stability
improvment when you are un the context of frequent seamless reloads with
a high "hard-stop-after" value; for that reasons I think this patch
should be backported in all 2.x versions.

Signed-off-by: William Dauchy <w.dauchy@criteo.com>