sort the chunks we want to backup to tape by inode, to gain some
speed on spinning disks. this is done per index, not globally.
costs a bit memory, but not too much, about 16 bytes per chunk which
would mean ~4MiB for a 1TiB index with 4MiB chunks.
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>