index_column_diff: add support for customizing cache size
index_column_diff: add support for customizing cache size

We can customize cache size by GRN_INDEX_COLUMN_DIFF_CACHE_SIZE_MAX.
This is for finding suitable cache size. It's not for users. It's for
developers.

kenhys/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga,groonga/groonga,naoa/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,naoa/groonga,komainu8/groonga,naoa/groonga,komainu8/groonga,komainu8/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,komainu8/groonga,groonga/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga