Slight speep up fetching the endgame table

Slight speep up fetching the endgame table

Replace calls to count(key) + operator[key] with a single call to find(key).
Replace the std::map with std::unordered_map which provide O(1) access,
although the map has a really small number of objects.

Test with [0..4] failed yellow:

TC	10+0.1
SPRT	elo0: 0.00  alpha: 0.05  elo1: 4.00  beta: 0.05
LLR	-2.96 [-2.94,2.94] (rejected)
Elo	1.01 [-0.87,3.08] (95%)
LOS	85.3%
Games	71860 [w:22.3%, l:22.2%, d:55.5%]
http://tests.stockfishchess.org/tests/view/5d5432210ebc5925cf109d61

Closes https://github.com/official-stockfish/Stockfish/pull/2269

No functional change