Move 50 moves counter to initiative.

simplify the usage of the 50 moves counter,
moving it frome the scale factor to initiative.

This patch was inspired by recent games where a blocked or semi-blocked position
was 'blundered', by moving a pawn, into a lost endgame. This patch improves this situation,
finding a more robust move more often.

for example (1s searches with many threads):
```
FEN 8/p3kp2/Pp2p3/1n2PpP1/5P2/1Kp5/8/R7 b - - 68 143

master:
      6 bestmove b5c7
      6 bestmove e7e8
     12 bestmove e7d8
    176 bestmove e7d7
patch:
      3 bestmove b5c7
      5 bestmove e7d8
    192 bestmove e7d7
```

fixes https://github.com/official-stockfish/Stockfish/issues/2620

the patch also tests well

passed STC
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 50168 W: 9508 L: 9392 D: 31268
Ptnml(0-2): 818, 5873, 11616, 5929, 848
https://tests.stockfishchess.org/tests/view/5ebb07287dd5693aad4e680b

passed LTC
LLR: 2.93 (-2.94,2.94) {-1.50,0.50}
Total: 7520 W: 981 L: 870 D: 5669
Ptnml(0-2): 49, 647, 2256, 760, 48
https://tests.stockfishchess.org/tests/view/5ebbff747dd5693aad4e6858

closes https://github.com/official-stockfish/Stockfish/pull/2666

Bench: 4395562