Before this change we used the same (relatively low limits) for server
side copy as we did for multipart uploads. It doesn't make sense to
use the same limits since no data is being downloaded or uploaded for
a server side copy.
This change introduces a new parameter --s3-copy-cutoff to control
when the switch from single to multipart server size copy happens and
defaults it to the maximum 5GB.
This makes server side copies much more efficient.
It also fixes the erroneous error when trying to set the modification
time of a file bigger than 5GB.
See #3778
0d10640aaa