[SPARK-2603][SQL] Remove unnecessary toMap and toList in converting Java collections to Scala collections JsonRDD.scala

In JsonRDD.scalafy, we are using toMap/toList to convert a Java Map/List to a Scala one. These two operations are pretty expensive because they read elements from a Java Map/List and then load to a Scala Map/List. We can use Scala wrappers to wrap those Java collections instead of using toMap/toList.

I did a quick test to see the performance. I had a 2.9GB cached RDD[String] storing one JSON object per record. I tried `sqlContext.jsonRDD(jsonData)` to see the performance. The job running for schema inference had one stage and there were 48 tasks. These tasks were executed sequentially.

```
Original:
Run 1: 1.5 min
Run 2: 1.4 min
Run 3: 1.5 min

With this change:
Run 1: 1.2 min
Run 2: 1.2 min
Run 3: 1.2 min
```

JIRA: https://issues.apache.org/jira/browse/SPARK-2603