librbd: deep-copy should accept a lower-bound for the destination snap_id

librbd: deep-copy should accept a lower-bound for the destination snap_id

For snapshot-based mirroring, we will want to prevent the modification of
snapshots below the last sync snapshot and to prevent the copying of data
below that lower-bound as well. This commit just adds the new parameter and
future commits will update the snapshot and object copy behavior.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>