Fix subgraph carving using bad _ids

We weren't updating the "_vertexId" when copying graphs, which really
confused how subgraph carving happens. Since we our fork of titan-es
preserves _ids between Titan and ES, we can just use ES's "_id" field
and get rid of "_vertexId".
Merge pull request #149 from erickt/fix-carving

Fix subgraph carving using bad _ids