ii: use TF-IDF like score for similar search

[kou]

Old Groonga uses int for score data type. Recent Groonga uses double for score data type. Now, we can use floating point number for score.

How about using TF-IDF like score for similar search? This change doesn't change matched records order. It just changes score values to smaller values (around 1.0) from larger values. For example, 419432 -> 1.11 and 209716 -> 0.55. Smaller values will be useful for combining score computed by other search methods.