p-s: clear idle state after sighup to continue asap

if there are unchecked packages after a sighup, try to rerun
this section as early as possible and finish the work

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: check load frequently and sleep if it exceeds threshold

tunable via [global] slave-load-max = 1.5
0 (default) disables
9.5 seems to be reasonable for 8 slaves running on 4 cores / 8 threads

Signed-off-by: Andreas Beckmann <anbe@debian.org>