Scale down endgame factor when shuffling

This patch decreases the endgame scale factor using the 50 moves counter.
Looking at some games with this patch, it seems to have two effects on
the playing style:

1) when no progress can be made in late endgames (for instance in fortresses
   or opposite bishops endgames) the evaluation will be largely tamed down
   towards a draw value.

2) more interestingly, there is also a small effect in the midgame play because
   Stockfish will panic a little bit if there are more than four consecutive
   shuffling moves with an advantage: the engine will try to move a pawn or to
   exchange a piece to keep the advantage, so the follow-ups of the position
   will be discovered earlier by the alpha-beta search.

passed STC:
LLR: 2.95 (-2.94,2.94) [0.50,4.50]
Total: 23017 W: 5080 L: 4805 D: 13132
http://tests.stockfishchess.org/tests/view/5d7e4aef0ebc59069c36fc74

passed LTC:
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 30746 W: 5171 L: 4911 D: 20664
http://tests.stockfishchess.org/tests/view/5d7e513d0ebc59069c36ff26

Pull request: https://github.com/official-stockfish/Stockfish/pull/2304

Bench: 4272173