migrate es to HTTP client and not TransportClient (on deprecation path)

[amcp]

At some point in the future, support for the Transport Client (native Java client) will be removed.
To mitigate, we need to use the HTTP API.

[sjudeng]

The current Java REST client in Elasticsearch 5.x is very low level. I'd recommend waiting until they release a higher level Java REST client before working on migration to avoid unnecessary work.

[amcp]

I agree it is extremely low level and would require major refactoring. We should wait for a higher level REST client. I just wanted to get ES lack of commitment to the transport client on our radar (so that we know to plan for it).

[pluradj]

Related dev mailing list discussion https://groups.google.com/d/msg/janusgraph-dev/F-zfIsuiIoQ/q_R-kkhHDgAJ

[sjudeng]

With merge of #79 JanusGraph now supports the low-level ES REST HTTP client. But keeping this issue open to track work on migrating to higher level HTTP client for ES (e.g. Jest or maybe eventually ES 6.x REST).

[sjudeng]

With merge of #289 JanusGraph now only supports the ES REST HTTP client and the Transport client is no longer support.