Change DeleteObjectPieces for deleting the segments' pointers of an
object in a reverse order.
Last segment: L
N: total number of segments
Deleting in reverse order is: L, n-1 to 0
Deleting in reverse order makes BeginDeleteObject usable to delete
partially uploaded objects that were interrupted (e.g. upload
cancellation).
With this change, the uplink upload cancellation, can be changed to use
BeginDeleteObject to cleanup already uploaded segments without having to
retrieve orders and dial every single node which stored a piece.
Ticket: https://storjlabs.atlassian.net/browse/V3-3525
Change-Id: Ieca6fd3801c4b71671811cb5f08a99d5146928a6
d5a60aec58