rgw: bucket_list_ordered loops until it gets a unique candidate

rgw: bucket_list_ordered loops until it gets a unique candidate

when we detect a duplicate common prefix, we need to loop until we get
the next unique candidate. we must add a new candidate for each shard,
or we won't visit it again and would miss later entries

Fixes: https://tracker.ceph.com/issues/44353
Signed-off-by: Casey Bodley <cbodley@redhat.com>