[storage] support index_read_last_map for multiple colum index.
[storage] support index_read_last_map for multiple colum index.

refs #455

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