api2/node/tasks: use TaskListInfoIterator instead of read_task_list

api2/node/tasks: use TaskListInfoIterator instead of read_task_list

this makes the filtering/limiting much nicer and readable

since we now have potentially an 'infinite' amount of tasks we iterate over,
and cannot now beforehand how many there are, we return the total count
as always 1 higher then requested iff we are not at the end (this is
the case when the amount of entries is smaller than the requested limit)

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