rgw: encode timestamp in pending olh info

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw, cls_rgw: improve olh atomicity

*not tested*
Add olh_tag attr to olh object, and use it instead of obj_tag as object
data may be colocated with the olh. Use the olh_tag to guard various
operations.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>