cls_rgw: add missing cls_cxx_create()

This was found during code read, not necessarily a real issue, however,
we counted on the object being created implicitly by setting xattrs on
it. Instead make sure the object is created.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: add dump_string_header()

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: send x-amz-version-id and x-amz-delete_marker header fields

send these fields on obj delete as defined by the api. This required
adding an index read so that we can identify whether an instance is
actually a delete marker.

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