server/worker_task: simplify task log writing

server/worker_task: simplify task log writing

instead of prerotating 1000 tasks
(which resulted in 2 writes each time an active worker was finished)
simply append finished tasks to the archive (which will be rotated)

page cache should be good enough so that we can get the task logs fast

since existing installations might have an 'index' file, we
still have to read tasks from there, but only if it exists

this simplifies the TaskListInfoIterator a good amount

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