react-sortable-hoc broken in ConcurrentMode

I haven't really made a reproduction case yet, but just enabling `ConcurrentMode` in my tree that includes `react-sortable-hoc` is sufficient to break it as it is used in my application.

Related to https://github.com/clauderic/react-sortable-hoc/issues/444.

When `ConcurrentMode` is enabled, the helpers that show up when dragging starts never disappear, and the SortableList also does not correctly update when a reorder does happen during a drag. After the drop, the list does reorder, but because the helper never disappears (and multiplies if you start multiple drags), they each get stuck on screen.