Add GraphSON 2 support for JanusGraph predicates #1060

[FlorianHockmann]

This adds GraphSON support for JanusGraph predicates (Text and Geo) which is necessary to use them in GLVs.

I only implemented this for GraphSON 2 as it looks like GraphSON 3 currently doesn't even work with the default TinkerPop predicates (P.within(), P.gt(), ...). At least the test I added for that (testTinkerPopPredicatesAsGraphSON) fails with GraphSON 3 independent of my code changes.
Would be good to hear from someone with more knowledge of JanusGraph I/O if I'm doing something wrong here. When there are more problems with GraphSON 3 then a separate PR for that might be a good idea but I marked this PR as WIP until that has been clarified.

Issue: #1060

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[FlorianHockmann]

I looked a bit more into the issue with GraphSON 3 and confirmed that it has nothing to do with predicates. JanusGraph just can't deserialize GraphSON 3 right now. The tests I added even failed for a traversal like g.V() as the resulting serialized RequestMessage wraps the Bytecode now in a map which wasn't the case for GraphSON 2:

{
    "requestId": {
        "@type": "g:UUID",
        "@value": "164c5471-60f3-4f54-9f02-17918e5fe60b"
    },
    "op": "bytecode",
    "processor": "traversal",
    "args": {
        "@type": "g:Map",
        "@value": [
            "gremlin",
            {
                "@type": "g:Bytecode",
                "@value": {
                    "step": [
                        [
                            "V"
                        ]
                    ]
                }
            }
        ]
    }
}

I created #1075 for GraphSON 3 support. So this PR is now ready for review.

[pluradj]

a couple small fixes to do, otherwise nice work on this @FlorianHockmann!

I verified this manually with Gremlin Server configured with GraphSONMessageSerializerV2d0 as the only serializer then connected from Gremlin Console using the same serializer in the remote-objects.yaml.

[pluradj]

need to include the JanusGraph copyright header

[pluradj]

Replace "traversal" with a constant GraphSONTokens.TRAVERSAL

[FlorianHockmann]

I just amended my commit to include the two changes you requested, @pluradj.

[pluradj]

LGTM, thanks @FlorianHockmann

[twilmes]

Very nice, thanks @FlorianHockmann