Add age to transposition table replacement scheme

[kelseyde]

New order of preference for replacement is:

• An empty entry.
• An entry with the same zobrist key and a depth less than or equal to the new entry.
• The oldest entry in the bucket, stored further back in the game and so less likely to be relevant.
• The entry with the lowest depth.

Versus 3.4.0 @ 10+0.1:

Finished game 2649 (Calvin DEV vs Calvin): * {No result}
Score of Calvin DEV vs Calvin: 1060 - 932 - 647  [0.524] 2639
...      Calvin DEV playing White: 544 - 448 - 328  [0.536] 1320
...      Calvin DEV playing Black: 516 - 484 - 319  [0.512] 1319
...      White vs Black: 1028 - 964 - 647  [0.512] 2639
Elo difference: 16.9 +/- 11.5, LOS: 99.8 %, DrawRatio: 24.5 %