Pick bestmove from the deepest thread.

STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 26930 W: 4441 L: 4214 D: 18275

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 7783 W: 1017 L: 876 D: 5890

No functional change in single thread mode

Resolves #485

src/thread.h

@@ -76,6 +76,7 @@ struct Thread : public ThreadBase {
   Depth rootDepth;
   HistoryStats history;
   MovesStats counterMoves;
+  Depth completedDepth;
 };