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

This reverts commit 9dff2a4012bb9d9803634ee0c98680282ae0461f.

It also affected PRIMARY KEY.
storage: read from _key only when PRIMARY KEY is referenced by other column

Because _key is normalized. Mroonga should return the
original (unnormalized) value if it is possible. The original value is
stored in column not _key.