Changes in TBB 2019 Update 6:

• Added support for enqueuing tbb::task into tbb::task_arena (https://github.com/01org/tbb/issues/116).
• Improved support for allocator propagation on concurrent_hash_map assigning and swapping.
• Improved scalable_allocation_command cleanup operations to release more memory buffered by the calling thread.
• Separated allocation of small and large objects into distinct memory regions, which helps to reduce excessive memory caching inside the TBB allocator.
• (Preview feature) Removed template class gfx_factory from the flow graph API.