Make round-robin task enqueue index thread safe

src/Yarn/Scheduler.cpp

@@ -99,6 +99,7 @@ void Scheduler::unbind()
 }
 
 Scheduler::Scheduler()
+    : nextEnqueueIndex(0)
 {
     for (size_t i = 0; i < spinningWorkers.size(); i++)
     {

src/Yarn/Scheduler.hpp

@@ -294,7 +294,7 @@ private:
 
     // TODO: Make this lot thread-safe so setWorkerThreadCount() can be called
     // during execution of tasks.
-    unsigned int nextEnqueueIndex = 0;
+    std::atomic<unsigned int> nextEnqueueIndex;
     unsigned int numWorkerThreads = 0;
     std::array<Worker*, MaxWorkerThreads> workerThreads;