Improve Fabric Indexing

[zachkinstner]

Update Fabric's indexing strategy to use vertex-centric indexes for appropriate edges. This improvement depends upon the outcome of a specific Titan change: https://github.com/thinkaurelius/titan/pull/287

Related discussion: https://groups.google.com/forum/#!topic/aureliusgraphs/KFVtfC9Xy94

[zachkinstner]

Remaining: implement a way to automatically set an edge's VCI properties during addEdge(). I don't want to re-write the code for this process every time/place an edge is created.

I'm thinking something like this:

• The addEdge() function always has references to out/in vertices.
• Instead of adding property values via Fabric code, use the properties of the vertex references.
• So maybe: addEdge(a, b, [a.Created, a.IndexedId, b.Something])

[zachkinstner]

This functionality belongs in Weaver.Titan.

[zachkinstner]

Completed in Weaver.Titan.