Restore development version

No functional change.
Use null move when depth >= 2 plies

Passed both short TC:
LLR: 2.96 (-2.94,2.94)
Total: 23725 W: 5031 L: 4855 D: 13839

And long TC:
LLR: 2.96 (-2.94,2.94)
Total: 15730 W: 2939 L: 2754 D: 10037

bench: 4729333
Retire redundant endgames

The case of two lone kings on the board is already considered
by the "No pawns" scaling factor rules in material.cpp as is
KBK and KNK.

Moreover we had a small leak in endgames map because for
KK endgame it happens white and black material keys are the
same (both equal to zero), so when adding the black endgame in
Endgames::add() we were overwriting the already exsisting
white one, leading to a memory leak found by Valgrind.

So remove the endgames althogheter and rely on scaling
to correctly set the endgames value to a draw.

No functional change.
Simplify condition for backward pawn

No functional changes.