Introduce yielding spin locks

Idea and original implementation by Stephane Nicolet

7 threads 15+0.05
ELO: 3.54 +-2.9 (95%) LOS: 99.2%
Total: 17971 W: 2976 L: 2793 D: 12202

There is no functional change in single thread mode

src/search.cpp

@@ -1696,7 +1696,7 @@ void Thread::idle_loop() {
               if (   sp->allSlavesSearching
                   && sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT)
               {
-                  mutex.lock();
+                  allocMutex.lock();
 
                   if (can_join(sp))
                   {
@@ -1705,7 +1705,7 @@ void Thread::idle_loop() {
                       searching = true;
                   }
 
-                  mutex.unlock();
+                  allocMutex.unlock();
               }
 
               sp->mutex.unlock();