indexes only work if added *after* loading the data

[mpollmeier]

the below works, but fails if we swap loading and index creation. check for both vertices and edges.

TinkerGraph graph = TinkerGraph.open(
    Arrays.asList(Song.factory, Artist.factory),
    Arrays.asList(FollowedBy.factory, SungBy.factory, WrittenBy.factory)
);
loadGraphMl(graph);
graph.createIndex("weight", Edge.class);
GraphTraversalSource g = graph.traversal();
long count = g.E().has("weight", P.eq(1)).count().next();
assertEquals(3564, count);