This change to the S3 Move method uses S3's multipart upload API to copy
objects whose size exceeds a threshold. Parts are copied concurrently.
The level of concurrency, part size, and threshold are all configurable
with reasonable defaults.
Using the multipart upload API has two benefits.
* The S3 Move method can now handle objects over 5 GB, fixing #886.
* Moving most objects, and espectially large ones, is faster. For
example, moving a 1 GB object averaged 30 seconds but now averages 10.
Signed-off-by: Noah Treuhaft <noah.treuhaft@docker.com>
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