Simplify locking in sp_search and sp_search_pv

Avoid to take the lock two times in a tight sequence, the first
in get_next_move() and the second to update sp->moves.

Do all with one lock and so retire the now useless locked version
of get_next_move().

Also fix some theorical race due to comparison sp->bestValue < sp->beta
is done out of lock protection. Finally fix another (harmless but time
waster) race that coudl occur because thread_should_stop() is also
called outside of lock protection.

No functional change.

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

src/lock.h

@@ -99,5 +99,9 @@ typedef CRITICAL_SECTION Lock;
 
 #endif
 
+static inline bool lock_grab_bool(Lock* x) {
+    lock_grab(x);
+    return true;
+}
 
 #endif // !defined(LOCK_H_INCLUDED)