fix #3526: correctly filter tasks with 'since' and 'until'

fix #3526: correctly filter tasks with 'since' and 'until'

The previous assumption was that the Tasks returned by the Iterator are
sorted by the starttime, but that is not actually the case, and
could never have been, since we append the tasks into the log when
they are finished (not started) and running tasks are always iterated
first.

To correctly filter (and simplify the the api call) we forgo the
combinators, and use a for loop instead. This way we only have to do
the since/until checks only once per Task, but have to do the
start/limit counting ourselves.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>