Implement a threefold repetition detection. Below are the examples of
problems fixed by this change.

    Loosing move in a drawn position.
    position fen 8/k7/3p4/p2P1p2/P2P1P2/8/8/K7 w - - 0 1 moves a1a2 a7a8 a2a1
    The old code suggested a loosing move "bestmove a8a7", the new code suggests "bestmove a8b7" leading to a draw.

    Incorrect evaluation (happened in a real game in TCEC Season 9).
    position fen 4rbkr/1q3pp1/b3pn2/7p/1pN5/1P1BBP1P/P1R2QP1/3R2K1 w - - 5 31 moves e3d4 h8h6 d4e3
    The old code evaluated it as "cp 0", the new code evaluation is around "cp -50" which is adequate.