MovePicker: combine countermove with killers.

Handle the countermove in the same way we use stages to progress
through the killer moves, using a common array called "refutations".
Removes some lines of code and simplifies a bit the jump table.

STC: LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 71707 W: 14622 L: 14595 D: 42490
http://tests.stockfishchess.org/tests/view/5aa003cf0ebc590297cb6276

LTC: LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 22320 W: 3470 L: 3352 D: 15498
http://tests.stockfishchess.org/tests/view/5aa051020ebc590297cb62ba

Closes https://github.com/official-stockfish/Stockfish/pull/1468

No functional change.