pvescheduler: make jobs tracking more flexible, rework stop

pvescheduler: make jobs tracking more flexible, rework stop

Avoid hard-coding the current implication of the replication stack to
not get started again until the old worker is done..

We still apply the same check, but changing that to let the jobs have
control is rather easy now.

Also rework the stop logic, send terminate to _all_ workers and make
the timeout a actual shared one (not first gets all, remaining get
kill) and send a kill to the stuck, leftover ones in one go at the
end, including some logging so that the admin can actually know about
this non-ideal situation.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>