Allow support for multiple serializer versions in Gremlin Server HTTP

The HttpGremlinEndPointHandler only deals with MessageTextSerializer implementations and you can't configure multiple GraphSON ones because all of those implementations bind to application/json mimetype. I'm not even quite sure why a distinction exists anymore between "text" and "binary" serialization because we serialize to string only to turn around and convert to a ByteBuf which is what the binary version of the serializers already do.

This is a breaking change because the header mime type appended to requests generated from GraphSONMessageSerializerV2d0 changed from application/json to application/vnd.gremlin-v2.0+json.