With the approach to graph node mutation on edge setting the previously
existed there was an issue that the edge last used connect a pair of
nodes could result in a difference in the nodes being returned by a node
query compared to the same node associated with edges returned from an
edge query.
This change avoids dealing with that by making it implementation
dependent and stating this, and by making all the node-storing graphs
we provide mutate the nodes when edges are set.
