Similarize pruning code in search() and sp_search()

Use futility pruning also in split points.
Do not use history pruning in split points when
getting mated.

After 1000 games on Joona QUAD
Orig - Mod: 496 - 504

Added an optimization to avoid a costly lock in the
very common case that sp->futilityValue <= sp->bestValue.
A test on a dual CPU shows only 114 hits on 23196 events,
so avoid a lock in all the other cases.

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