Rewrite early stop logic

Rewrite early stop logic

In the "easy move" paradigm we verify if the best move has
ever changed over a good number of iterations and if the
biggest part of the searched nodes are consumed on it.
This is a kind of hacky ad indirect heuristic to deduce
if one move is much better than others.

Rewrite the early stop condition to verify directly if one
move is much better than others performing an exclusion
search.

Idea to use exclusion search for time management if form Critter.

After 12245 games at 30"+0.1
Mod vs Orig 1776 - 1775 - 8694 ELO +0 (+-3.4)

Signed-off-by: Marco Costalba <mcostalba@gmail.com>