Now using the utility function for deduplication that was newly implemented to
fix an issue with server-side copy. This function uses the original, and generic,
"jfs" api (and its "cphash" feature), instead of the newer "allocate" api dedicated
for uploads. Both apis support similar deduplication functionaly that we rely on for
the SetModTime operation. One advantage of using the jfs variant is that the allocate
api is specialized for uploads, an initial request performs modtime-only changes and
deduplication if possible but if not possible it creates an incomplete file revision
and returns a special url to be used with a following request to upload missing content.
In the SetModTime function we only sent the first request, using metadata from existing
remote file but different timestamps, which lead to a modtime-only change. If, for some
reason, this should fail it would leave the incomplete revision behind. Probably not
a problem, but the jfs implementation used with this commit is simpler and
a more "standalone" request which either succeeds or fails without expecting additional
requests.
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