SPARK-4963 [SQL] Add copy to SQL's Sample operator

https://issues.apache.org/jira/browse/SPARK-4963
SchemaRDD.sample() return wrong results due to GapSamplingIterator operating on mutable row.
HiveTableScan make RDD with SpecificMutableRow and SchemaRDD.sample() will return GapSamplingIterator for iterating. 

override def next(): T = {
    val r = data.next()
    advance
    r
  }

GapSamplingIterator.next() return the current underlying element and assigned it to r.
However if the underlying iterator is mutable row just like what HiveTableScan returned, underlying iterator and r will point to the same object.
After advance operation, we drop some underlying elments and it also changed r which is not expected. Then we return the wrong value different from initial r.

To fix this issue, the most direct way is to make HiveTableScan return mutable row with copy just like the initial commit that I have made. This solution will make HiveTableScan can not get the full advantage of reusable MutableRow, but it can make sample operation return correct result.
Further more, we need to investigate  GapSamplingIterator.next() and make it can implement copy operation inside it. To achieve this, we should define every elements that RDD can store implement the function like cloneable and it will make huge change.

Use `checkAnswer` instead here as it give better output when there is an exception or the answer is wrong.

```scala
(1 to 10).foreach { i =>
  checkAnswer(
    sql("SELECT * FROM sampled WHERE key % 2 = 1"),
    Seq.empty)
}
```