tape/restore: optimize chunk restore behaviour

tape/restore: optimize chunk restore behaviour

by checking the 'checked_chunks' before trying to write to disk
and by doing the existance check in the parallel handler. This way,
we do not have to check the existance of a chunk multiple times
(if multiple source datastores gets restored to the same target
datastore) and also we do not have to wait on the stat before reading
the next chunk.

We have to change the &WorkerTask to an Arc though, otherwise we
cannot log to the worker from the parallel handler

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