Variable null-move value based reduction

Instead of a fixed reduction of ONE_PLY, now
Null move dynamic reduction based on value can
grow larger in case we are above beta of a value
much higher then PawnValueMg.

Note that now an eval returning VALUE_KNOWN_WIN
makes null search to drop in qsearch.

Passed both short TC:
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 26141 W: 4871 L: 4699 D: 16571

And long TC:
LLR: 2.97 (-2.94,2.94) [0.00,6.00]
Total: 33695 W: 5309 L: 5056 D: 23330

bench: 7356053

src/search.cpp

@@ -628,12 +628,12 @@ namespace {
     {
         ss->currentMove = MOVE_NULL;
 
-        // Null move dynamic reduction based on depth
-        Depth R = 3 * ONE_PLY + depth / 4;
+        assert(eval - beta >= 0);
 
-        // Null move dynamic reduction based on value
-        if (eval - PawnValueMg > beta)
-            R += ONE_PLY;
+        // Null move dynamic reduction based on depth and value
+        Depth R =  3 * ONE_PLY
+                 + depth / (2 * ONE_PLY)
+                 + int(eval - beta) / PawnValueMg * ONE_PLY;
 
         pos.do_null_move(st);
         (ss+1)->skipNullMove = true;
@@ -653,7 +653,8 @@ namespace {
 
             // Do verification search at high depths
             ss->skipNullMove = true;
-            Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
+            Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO)
+                                        :  search<NonPV>(pos, ss, alpha, beta, depth-R, false);
             ss->skipNullMove = false;
 
             if (v >= beta)