batch import support for janusgraph

There is a `BatchGraph` used to support batch import in TiTan, but seems this is removed in janusgraph, is there a dev plan to add batch import support in janusgraph?

This is a [bulk-loading](http://docs.janusgraph.org/latest/bulk-loading.html) documentation, but waiting for the real example for this case, thanks.

The issue names no implementation file or test. Start by reviewing the BatchGraph reference in TiTan and JanusGraph's linked bulk-loading documentation, then clarify whether the goal is implementation or a real example. Completion would need a defined, working batch-import capability or documented example.

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