ii: optimize for small result set (but disabled by default)

If result set is small and indexed records are large, searching with
index is slower than sequential search. This change uses sequential
search for the case.

You can custom the behavior by GRN_II_SELECT_TOO_MANY_INDEX_MATCH_RATIO
environment variable:

  * -1: Disable the optimization
  * other value: Use the optimization if
      ("# of result set" / "# of indexed records of a term") > GRN_II_SELECT_TOO_MANY_INDEX_MATCH_RATIO

The default GRN_II_SELECT_TOO_MANY_INDEX_MATCH_RATIO is -1. It means
that the feature is disabled by default.

Limitations:

  * CMake build isn't supported yet.
  * Support only exact match.