Caused by: java.lang.IllegalStateException: Multiple properties exist for the provided key, use Vertex.properties(lang)

**When I bulk load json data to janusgraph with BulkLoadVertexProgram（IncrementalBulkLoader），The problem has been encountered.
I want to know why this exception is thrown when the property value is more than two**

**tools version**
janusgraph-0.3.1
hadoop-gremlin-3.3.0
hbase-1.1.3
es-6.2.4

**janusgraph schema**
```java
JanusGraphManagement m = janusGraph.openManagement()
VertexLabel person = m.makeVertexLabel("person").make()
VertexLabel software = m.makeVertexLabel("software").make()
PropertyKey blid = m.makePropertyKey("bulkLoader.vertex.id").dataType(Long.class).make()
PropertyKey age = m.makePropertyKey("age").dataType(Integer.class).cardinality(Cardinality.LIST).make()
PropertyKey name = m.makePropertyKey("name").dataType(String.class).cardinality(Cardinality.SET).make()
PropertyKey lang = m.makePropertyKey("lang").dataType(String.class).cardinality(Cardinality.LIST).make()
m.addProperties(person, blid, name, age)
m.addProperties(software, blid, name, lang)
EdgeLabel created = m.makeEdgeLabel("created").make()
EdgeLabel knows = m.makeEdgeLabel("knows").make()
PropertyKey weight = m.makePropertyKey("weight").dataType(Float.class).make()
m.addProperties(created, weight)
m.addProperties(knows, weight)
//index
JanusGraphIndex index = m.buildIndex("mixedIndex", Vertex.class).addKey(name, Mapping.TEXT.asParameter()).buildMixedIndex("search")//不支持唯一性检查，search为index.search.backend中的search
//使用IncrementBulkLoader导入时，去掉下面注释
JanusGraphIndex bidIndex = m.buildIndex("byBulkLoaderVertexId", Vertex.class).addKey(blid).indexOnly(person).buildMixedIndex("search")
m.commit()
```

**my code**
```java
JanusgraphSchema.defineGratefulDeadSchema(janusGraph);
Graph graph = GraphFactory.open("config/hadoop-graphson.properties");
BulkLoaderVertexProgram blvp = BulkLoaderVertexProgram.build().bulkLoader(IncrementalBulkLoader.class).writeGraph("config/janusgraph-hbase-es.properties").create(graph);
graph.compute(SparkGraphComputer.class).program(blvp).submit().get();
graph.close();
GraphTraversalSource g = janusGraph.traversal();
List> vertexList = g.V().valueMap().toList();
System.out.println("data1: " + vertexList);
janusGraph.close();
```

Start with BulkLoaderVertexProgram and IncrementalBulkLoader, then trace how the schema's LIST and SET cardinalities are handled before the valueMap() call. Reproduce the exception with the supplied JanusGraph, Hadoop-Gremlin, HBase, and Elasticsearch versions; done means determining whether the behavior is expected and documenting or correcting the handling of multiple property values.

Written by the indexing model from the issue text.