datastore: add tuning option for chunk order

datastore: add tuning option for chunk order

currently, we sort chunks by inode when verifying or backing up to tape.
we get the inode# by stat'ing each chunk, which may be more expensive
than the gains of reading the chunks in order

Since that is highly dependent on the underlying storage of the datastore,
introduce a tuning option  so that the admin can tune that behaviour
for each datastore.

The default stays the same (sorting by inode)

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