Make GRN_JA_SEGREGATE_THRESHOLD customizable

  * configure: --with-ja-segregate-threshold=16
  * CMake: -DGRN_JA_SEGREGATE_THRESHOLD=16

Because changing segregate threshold in ja breaks database
compatibility. We can't introduce the breakage in 4.0.1.

But this change will be usable for many users. So we provide a option
to customize the value.