rgw: use bucket_info.bucket_id instead of marker where needed

rgw: use bucket_info.bucket_id instead of marker where needed

We used to use these interchangeably, but they actually have diffent meaning.
The marker is the prefix we assign to the objects in this bucket, whereas
the bucket_id is the bucket instance's id. These used to hold the same
value, but with bucket resharding it's not going to be true anymore.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
(cherry picked from commit 94afaf722fc8289af6e75025ba7d783f11c4b7d0)
See: http://tracker.ceph.com/issues/17556
See: https://github.com/ceph/ceph/pull/11368
Signed-off-by: Robin H. Johnson <robin.johnson@dreamhost.com>