Optimized the non blocking thread pool:

* Use a pseudo-random permutation of queue indices during random stealing. This ensures that all the queues are considered.
 * Directly pop from a non-empty queue when we are waiting for work,
instead of first noticing that there is a non-empty queue and
then doing another round of random stealing to re-discover the non-empty
queue.
 * Steal only 1 task from a remote queue instead of half of tasks.

unsupported/Eigen/CXX11/src/ThreadPool/RunQueue.h

@@ -100,7 +100,7 @@ class RunQueue {
   // PopBack removes and returns the last elements in the queue.
   // Can fail spuriously.
   Work PopBack() {
-    if (Empty()) return 0;
+    if (Empty()) return Work();
     std::unique_lock<std::mutex> lock(mutex_, std::try_to_lock);
     if (!lock) return Work();
     unsigned back = back_.load(std::memory_order_relaxed);