[ML] Regression job failed in coarse_parameter_search phase if dependent_variable field is unsigned_long

When testing `unsigned_long` data type in 7.10, DFA regression job failed in coarse_parameter_search phase if dependent_variable is `unsigned_long`,

**Error in es log:**
```
[2020-10-30T00:03:25,932][INFO ][o.e.x.m.d.p.AnalyticsProcessManager] [Weis-MacBook-Pro.local] [wine_ul] Started analyzing
[2020-10-30T00:03:25,932][INFO ][o.e.x.m.d.p.AnalyticsProcessManager] [Weis-MacBook-Pro.local] [wine_ul] Waiting for result processor to complete
[2020-10-30T00:03:26,172][INFO ][o.e.x.m.p.AbstractNativeProcess] [Weis-MacBook-Pro.local] [wine_ul] State output finished
[2020-10-30T00:03:26,172][ERROR][o.e.x.m.p.AbstractNativeProcess] [Weis-MacBook-Pro.local] [wine_ul] analytics process stopped unexpectedly:
[2020-10-30T00:03:26,173][ERROR][o.e.x.m.p.l.CppLogMessageHandler] [Weis-MacBook-Pro.local] [controller/5526] [CDetachedProcessSpawner.cc@184] Child process with PID 9510 was terminated by signal 11
[2020-10-30T00:03:26,173][ERROR][o.e.x.m.p.l.CppLogMessageHandler] [Weis-MacBook-Pro.local] [controller/5526] [CDetachedProcessSpawner.cc@99] Will not attempt to kill process 9510: not a child process
[2020-10-30T00:03:26,173][ERROR][o.e.x.m.p.l.CppLogMessageHandler] [Weis-MacBook-Pro.local] [controller/5526] [CCommandProcessor.cc@115] Failed to kill process with PID 9510 in command with ID 524
[2020-10-30T00:03:26,174][ERROR][o.e.x.m.d.p.AnalyticsProcessManager] [Weis-MacBook-Pro.local] [wine_ul] Failed to kill process
java.io.IOException: ML controller failed to execute command [524]: [Failed to kill process with PID 9510]
```

**Job configure:**
```
{
"id": "wine_ul",
"create_time": 1604030604645,
"version": "8.0.0",
"description": "",
"source": {
"index": [
"wine_ul"
],
"query": {
"match_all": {}
}
},
"dest": {
"index": "dest_wine_ul",
"results_field": "ml"
},
"analysis": {
"regression": {
"dependent_variable": "quality",
"num_top_feature_importance_values": 0,
"prediction_field_name": "quality_prediction",
"training_percent": 95,
"randomize_seed": 95,
"loss_function": "mse"
}
},
"analyzed_fields": {
"includes": [
"alcohol",
"chlorides",
"citric acid",
"density",
"fixed acidity",
"free sulfur dioxide",
"pH",
"quality",
"residual sugar",
"sulphates",
"total sulfur dioxide",
"volatile acidity"
],
"excludes": []
},
"model_memory_limit": "125mb",
"allow_lazy_start": false,
"max_num_threads": 1
}
```

dataset csv file can be found here:
[wine_quality_white_ul.csv.zip](https://github.com/elastic/ml-cpp/files/5623518/wine_quality_white_ul.csv.zip)

Step to reproduce:
- use data viz to import wine_quality_white_ul.csv file
- manual update the `quality` mapping to `unsigned_long`
- create a DFA job by using the above job configure
- Run job

wine_quality_white_ul.csv、提供されたジョブ設定、および unsigned_long の品質マッピングを使用して障害を再現します。まず、coarse_parameter_search フェーズと Elasticsearch ログ内のネイティブプロセスのクラッシュを調査します。dependent_variable が unsigned_long の場合に DFA 回帰ジョブが正常に完了すれば完了です。

索引モデルが issue の本文から書いたものです。