Revert "storage: don't use _key for UNIQUE KEY"
Revert "storage: don't use _key for UNIQUE KEY"

This reverts commit 9dff2a4012bb9d9803634ee0c98680282ae0461f.

It also affected PRIMARY KEY.

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