More random draw evaluations

Use the randomized draw function value_draw() also for draw evalutions.

This extends the earlier commit
97d2cc9
which did this only for 3folds.

As in that case, this test was yellow at STC and LTC, but green at VLTC,
indicative of the fact that the higher the drawrate, the more likely this
idea is beneficial.

STC:
LLR: -2.96 (-2.94,2.94) [0.50,4.50]
Total: 83573 W: 18584 L: 18335 D: 46654
http://tests.stockfishchess.org/tests/view/5d84e44d0ebc5971531d4f94

LTC:
LLR: -2.96 (-2.94,2.94) [0.00,3.50]
Total: 92252 W: 15240 L: 15160 D: 61852
http://tests.stockfishchess.org/tests/view/5d865dd90ebc5971531d68e1

VLTC: 120+1.2 @ 2th
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 51902 W: 7323 L: 7028 D: 37551
http://tests.stockfishchess.org/tests/view/5d8763620ebc595f57c22b15

Closes #2321

Bench: 3441237

src/search.cpp

@@ -762,6 +762,9 @@ namespace {
         if (eval == VALUE_NONE)
             ss->staticEval = eval = evaluate(pos);
 
+        if (eval == VALUE_DRAW)
+            eval = value_draw(depth, thisThread);
+
         // Can ttValue be used as a better position evaluation?
         if (    ttValue != VALUE_NONE
             && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))