The S3 ListObjects response from `rclone serve s3` was sorting object
contents by modification time instead of object key. This made the
listing order incompatible with S3 clients which expect lexicographic
key ordering.
In particular, `aws s3 sync` assumes both source and destination
iterators are ordered by key. With the old modtime ordering it could
misidentify files as missing or outdated and re-download objects that
were already up to date.
Change the pager to sort returned objects by key and add a regression
test which uses keys and modtimes arranged so the old behaviour would
fail.
Fixes#9002