fix: too many vertices for one time post in spark-loader

[simon824]

The option batch.max_vertices_per_batch default value is 500, if spark/flink default value of batch_size is bigger than 500 ,will cause the below exception.

class java.lang.IllegalArgumentException: Too many vertices for one time post, the maximum number is '500'
  at com.baidu.hugegraph.exception.ServerException.fromResponse(ServerException.java:47)
  at com.baidu.hugegraph.client.RestClient.checkStatus(RestClient.java:93)
  at com.baidu.hugegraph.rest.AbstractRestClient.post(AbstractRestClient.java:231)
  at com.baidu.hugegraph.rest.AbstractRestClient.post(AbstractRestClient.java:211)
  at com.baidu.hugegraph.api.graph.VertexAPI.create(VertexAPI.java:58)
  at com.baidu.hugegraph.driver.GraphManager.addVertices(GraphManager.java:96)
  at com.baidu.hugegraph.loader.spark.HugeGraphSparkLoader.flush(HugeGraphSparkLoader.java:253)
  at com.baidu.hugegraph.loader.spark.HugeGraphSparkLoader.loadRow(HugeGraphSparkLoader.java:150)
  at com.baidu.hugegraph.loader.spark.HugeGraphSparkLoader.lambda$null$0(HugeGraphSparkLoader.java:111)

closed #311

[codecov]

Codecov Report

Merging #312 (a2905c7) into master (a594254) will decrease coverage by 0.33%.
The diff coverage is 2.85%.

@@             Coverage Diff              @@
##             master     #312      +/-   ##
============================================
- Coverage     67.83%   67.49%   -0.34%     
  Complexity      877      877              
============================================
  Files            86       86              
  Lines          4004     4024      +20     
  Branches        474      475       +1     
============================================
  Hits           2716     2716              
- Misses         1084     1104      +20     
  Partials        204      204              

Impacted Files	Coverage Δ
.../hugegraph/loader/flink/HugeGraphOutputFormat.java	0.00% <0.00%> (ø)
...u/hugegraph/loader/spark/HugeGraphSparkLoader.java	0.00% <0.00%> (ø)
...baidu/hugegraph/loader/mapping/ElementMapping.java	87.83% <100.00%> (ø)

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[javeme]

LGTM