Untested and not even compiled yet.
Testing should include checks that file content streams through without
buffering in memory.
Note that CL.consume causes all the etags to be buffered in memory.
This is probably nearly unavoidable, since a request has to be constructed
that contains the list of etags in its body. (While it might be possible to
stream generation of the body, that would entail making a http request that
dribbles out parts of the body as the multipart uploads complete, which is
not likely to work well..
To limit this being a problem, it's best for partsize to be set to some
suitably large value, like 1gb. Then a full terabyte file will need only
1024 etags to be stored, which will probably use around 1 mb of memory.
