server/worker_task: split task list file into two

server/worker_task: split task list file into two

one for only the active tasks and one for up to 1000 finished tasks

factor out the parsing of a task file (we will later need this again)
and use iterator combinators for easier code

we now sort the tasks ascending (this will become important in a later patch)
but reverse (for now) it to keep compatibility

this code also omits the converting into an intermittent hash
since it cannot really happen that we have duplicate tasks in this list
(since the call is locked by an flock, and it is the only place where we
write into the lists)

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