Store moves statistics per board and use them for AI

[portnov]

For each board, store:

• how many times did we encounter this board in a real game
• sum of all scores that we had in that games by the end
• minimum and maximum scores

Questions:

• do we need to store this information for each depth value? - no, we dont. We will count only end results of the game.
• store this information in the same file as cache, or in separate one? - let's start from single file.
• How exactly to use this information? Options:

a) If for this board we have enough statistics, then use statistics, otherwise try to use cache or calculate score.
b) First calculate score; then if some moves lead to equal score, then check for statistics.

[portnov]

For initial board, we obviously will have minScore = loose, maxScore = win. This does not say anything.
So it only makes sense to use average score, and only when number of games is large enough. Possible criteria: number of games > number of possible moves.