Add Dgraph implementation

[szarnyasg]

Idea for loading: https://wiki.dgraph.io/Query_Language#Batch_mutations

[szarnyasg]

Branch: https://github.com/FTSRG/trainbenchmark/tree/dgraph

Tutorials:

• http://graphql.org/learn/queries/
• https://docs.dgraph.io/master/query-language/

(Update.) I made this issue part of #101.

[tzdybal]

Don't look at http://graphql.org/learn/queries/ - Dgraph's GraphQL+- is a different language.
You better use https://tour.dgraph.io/ to quickly learn how to work with Dgraph.

[szarnyasg]

Thanks @tzdybal! I pinged you on the Neo4j Slack, please take a look if you have a chance.

[manishrjain]

Hey @szarnyasg , if this is a one time load, then might be worth using the bulk loader: https://docs.dgraph.io/deploy/#bulkloader, which just got released in v0.8.2.

And if you run into any issues, or have any questions, please do feel free to reach out to us at slack.dgraph.io. We'd love to review the queries, etc. to ensure that they're optimized.