in_values: support auto sequential search mode

It will be faster when almost records are matched with index search.

It is used when

    (n_matched_records / n_index_searchable_records) > 0.01

"0.01" can be customizable by `GRN_IN_VALUES_TOO_MANY_INDEX_MATCH_RATIO`
environment variable.

For example, `GRN_IN_VALUES_TOO_MANY_INDEX_MATCH_RATIO=1.0` means that
sequential search is used rather than index search when the number of
matched records is less than or equal to the number of index searchable
records.

You can disable sequential search by specifying negative value to
`GRN_IN_VALUES_TOO_MANY_INDEX_MATCH_RATIO` like:

    GRN_IN_VALUES_TOO_MANY_INDEX_MATCH_RATIO=-1

Limitations:

  * Support only reference type column. It means that sequential search
    isn't used for `ShortText` type column.
  * Support only single column index.
  * Support only index that doesn't use weight.
  * Support only AND logical operator.