rgw: guard against racing writes

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: more atomicity fixes, set_olh()

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: tie set_olh() to object completion

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
radosgw-admin: a fix (squash)

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