The upload_sink::_upload_id remains empty until upload starts, remains
non-empty while it proceeds, then becomes empty again after it
completes. The upload_started() method cheks that and on .close()
started upload is aborted.
The final switch to empty is done by std::move()ing the upload id into
completion requrest, but it's better to use std::exchange() to emphasize
the fact the the _upload_id becomes empty at that point for a reason.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closes#13570
30b6f34a0b