Initialize a new split point out of lock

Allocate and initialize a new split point
out of lock becuase modified data is local to
master thread only.

Also better document why we need a lock at
the end of split().

No functional change with faked split.

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

src/thread.h

@@ -119,7 +119,7 @@ public:
              Depth depth, Move threatMove, int moveCount, MovePicker* mp, int nodeType);
 private:
   Thread threads[MAX_THREADS];
-  Lock mpLock;
+  Lock threadsLock;
   Depth minimumSplitDepth;
   int maxThreadsPerSplitPoint;
   int activeThreads;