v1.4.6 of uplink allows us to do a negative offset from the end of the
file. This removes a round trip when requesting the last N bytes of a
file.
Previous to v1.4.6 of uplink it wasn't possible to do a negative offset
on download. This meant that to fulfill the semantics of http range
headers it was necessary to first fetch the size of the object via a
stat call and compute absolute offset and length.
