Implement many-to-many edges

[schrockn]

This will be necessary for many application graphs. A good example of many-to-many edges is twitter's follow graph (directional) or facebook friend graph (bidirectional). One challenge will be managing which edge is "canonical". An initial strategy would be to have the id that is "lower" than the other on the edge determine sharding and have the other id be extra data. However that may have side effects I don't anticipate.