range(low, high) doesn't work with mixed index

Nobody has claimed this yet.

Following on #252, it has been noted that `order().by(property).range(low, high)` does not take advantage of the mixed index. @davidclement90 [noted](https://github.com/JanusGraph/janusgraph/issues/252#issuecomment-353629881):

> IndexQuery do not support offset. So it just support ranging with no offset range(0,10). Range with offset will be do in-memory with the high range as query limit. https://github.com/JanusGraph/janusgraph/blob/a127060de7125a94261cc313697ce778ac7ec22d/janusgraph-core/src/main/java/org/janusgraph/graphdb/tinkerpop/optimize/HasStepFolder.java#L213

Also more recently on [janusgraph-users](https://groups.google.com/d/msg/janusgraph-users/jUNvyQ0ohbw/Mbet-sdAAAAJ):

Query:
```
g.V().hasLabel("location").
has("location_type", 3).
order().by("location_id").
range(100000, 111000).toList()
```
Result: 8 minutes

Start with HasStepFolder.java around the linked line and trace how IndexQuery handles range offsets. Reproduce the Gremlin query from the issue against a mixed index, then verify that range(100000, 111000) uses the index rather than relying on a long in-memory operation.

Written by the indexing model from the issue text.