- use of TBB task priority is re-enabled (albeit deprecated)

- replaced task::spawn with task::enqueue with priority_high due to
  - somehow spawned task never getting picked up when doing seemingly trivial TiledArray example in ValeevGroup/tiledarray#195 when MADNESS uses TBB and OpenMPI (3 or 4):
  - https://software.intel.com/en-us/forums/intel-threading-building-blocks/topic/326043 suggested that this is due to wait_for_all not getting called, but in the failing example main does get to fence and calls wait_for_all ... perhaps since this may be a different thread??

src/madness/world/thread.h

@@ -1324,11 +1324,12 @@ namespace madness {
             __dague_schedule(parsec->virtual_processes[0]->execution_units[0], context);
             //////////// Parsec Related End ////////////////////
 #elif HAVE_INTEL_TBB
-            if(task->is_high_priority()) {
-                tbb::task::spawn(*task);
-            } else {
+#ifdef MADNESS_CAN_USE_TBB_PRIORITY
+            if(task->is_high_priority())
+                tbb::task::enqueue(*task, tbb::priority_high);
+            else
+#endif  // MADNESS_CAN_USE_TBB_PRIORITY
                 tbb::task::enqueue(*task);
-            }
 #else
             if (!task) MADNESS_EXCEPTION("ThreadPool: inserting a NULL task pointer", 1);
             int task_threads = task->get_nthread();

src/madness/world/worldrmi.cc

@@ -402,7 +402,11 @@ namespace madness {
                 new (tbb::task::allocate_root()) tbb::empty_task;
             tbb_rmi_parent_task->set_ref_count(2);
             task_ptr = new (tbb_rmi_parent_task->allocate_child()) RmiTask(comm);
+#ifdef MADNESS_CAN_USE_TBB_PRIORITY
+            tbb::task::enqueue(*task_ptr, tbb::priority_high);
+#else
             tbb::task::enqueue(*task_ptr);
+#endif  // MADNESS_CAN_USE_TBB_PRIORITY
 
             task_ptr->comm.Barrier();
 
@@ -424,6 +428,9 @@ namespace madness {
               for (int i = 0; i < NEMPTY; i++) {
                 tbb::task* empty =
                     new (empty_root->allocate_child()) tbb::empty_task;
+#ifdef MADNESS_CAN_USE_TBB_PRIORITY
+                tbb::task::enqueue(*empty, tbb::priority_high);
+#else
                 tbb::task::enqueue(*empty);
 #endif  // MADNESS_CAN_USE_TBB_PRIORITY
                 ++binge_counter;