Deleting steps while looping through the steps on a node

[adamnovak]

I want to loop over the steps on a node with HashGraph::for_each_step_on_handle_impl and delete their corresponding paths with HashGraph::destroy_path. However, if I do this, the reordering of the steps on the node that the delete does will not be picked up on by the iteration, and I won't necessarily visit all the steps on the node.

Either we need to make this work, or we need to specify that delete operations are not permitted during iterations over things they might change.