mariadb10.1 storage: support index_flags() for ORDER BY DESC
mariadb10.1 storage: support index_flags() for ORDER BY DESC

key::user_defined_key_parts is used as part. We may use the behavior for
optimization in the future.

naoa/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,mroonga/mroonga,mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga