bugfix: Refutation::ckear() was broken (6902519)
sizeof(this) was not understood properly by the compiler (it would have been if count was #defined rather than a static const int).
only the first element of the table was erased, so the refutation table was never really cleared.
clearing it before every new search could be a regression, so only clear it after ucinewgame.

After a call to search::clear_state(), the search results will be reproductible. so do that after ucinewgame, and in between test positions in bench(). Of course, doing it before every new search could be regressive, so do not commit (need to do some testing).
bugfix: update bestmove when fail low at root
no search::clear_state() between bench positions (6675476)