New feauture: Add --stale-routes-check

New feauture: Add --stale-routes-check

This patch adds a stale-routes-check option that takes 2 parameters: a ageing
time (in seconds) and a check interval (in seconds). The latter defaults to the
former if it's not present.  Internally, a new "check" is added in
multi_process_per_second_timers_dowork(). This check deletes stale routes and
it is inspired to the function multi_reap_range().

We're running a very large connectivity infrastructure based on openVPN (more
than 4000 different clients connected per day per server), so we can throughly
check this patch (or, of course, any variant of it).

Signed-off-by: Davide Guerri <d.guerri@caspur.it>
Reviewed-by: David Sommerseth <davids@redhat.com>
Acked-by: Adriaan de Jong <dejong@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Signed-off-by: David Sommerseth <davids@redhat.com>