tape/pool_writer: skip already backed up chunks in iterator

tape/pool_writer: skip already backed up chunks in iterator

currently, the iterator goes over *all* chunks of the index, even
those already backed up by a previous snapshots in the same tape
backup. this is bad since for each iterator, we stat each chunk to
sort by inode number. so to avoid stat'ing the same chunks over
and over for consecutive snapshots, add a 'skip_fn' to the iterator
and in the pool writer and check the catalog_set if we can skip it

this means we can drop the later check for the catalog_set
(since we don't modify that here)

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