rgw: allow ordered bucket listing to work when many filtered out entries

rgw: allow ordered bucket listing to work when many filtered out entries

A previous PR moved the much of the filtering that's part of bucket
listing to the CLS layer. One unanticipated result was that it is now
possible for a call to return 0 entries. In such a case we want to
retry the call with the marker moved forward (i.e., advanced),
repeatedly if necessary, in order to either retrieve some entries or
to hit the end of the entries. This PR adds that functionality.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>