Create a graph changefeed. When edges are created, destroyed, or updated
publish a changeset via Redis PubSub. Each changeitem contains two
fields: "before" and "after". Each changeset is an array of changeitems.
Currently only destroyed a graph node will use the batch nature of
changesets, whereas other graph mutations are published as single-item
changesets. By exposing changesets now we make future batching
optimizations possible without affecting the API.