Active Reparenting

In Young Brothers Wait Concept (YBWC) available slaves are
booked by the split point master, then start to search below
the assigned split point and, once finished, return in idle
state waiting to be booked by another master.

This patch introduces "Active Reparenting" so that when a
slave finishes its job on the assigned split point, instead
of passively waiting to be booked, searches a suitable active
split point and reprents itselfs to that split point. Then
immediately starts to search below the split point in exactly
the same way of the others split point's slaves. This reduces
to zero the time waiting in idle loop and should increase
scalability especially whit many (8 or more) cores.

No functional change.

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

src/thread.h

@@ -51,6 +51,7 @@ struct SplitPoint {
   // Shared data
   Lock lock;
   volatile uint64_t slavesMask;
+  volatile uint64_t allSlavesMask;
   volatile int64_t nodes;
   volatile Value alpha;
   volatile Value bestValue;