Use pipe between ceph backup diff export/import

We now use a piped transfer between the rbd export-diff
and import-diff for incremental backups/restores as
opposed to holding the entire diff in memory.

Change-Id: I33476d9b3934781413af5cd2867a11d825a5d78e
Fixes: bug 1244464
Merge "Use pipe between ceph backup diff export/import"