Ceph's Swift API emulation does not fully confirm to the API spec.
As a result, it sometimes returns fewer items in a container than
the requested limit, which according to the spec should means
that there are no more objects left in the container. (Note that
python-swiftclient always fetches unless the current page is empty.)
This commit adds a pair of new Swift backend settings to handle this.
Set `fetch_until_empty_page` to true to always fetch another
page of the container listing unless there are no items left.
Alternatively, set `partial_page_fetch_threshold` to an integer
percentage. In this case rclone will fetch a new page only when
the current page is within this percentage of the limit.
Swift API reference: https://docs.openstack.org/swift/latest/api/pagination.html
PR against ncw/swift with research and discussion: https://github.com/ncw/swift/pull/167Fixes#7924
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