[SPARK-14063][SQL] SQLContext.range should return Dataset[java.lang.Long]

## What changes were proposed in this pull request?
This patch changed the return type for SQLContext.range from `Dataset[Long]` (Scala primitive) to `Dataset[java.lang.Long]` (Java boxed long).

Previously, SPARK-13894 changed the return type of range from `Dataset[Row]` to `Dataset[Long]`. The problem is that due to https://issues.scala-lang.org/browse/SI-4388, Scala compiles primitive types in generics into just Object, i.e. range at bytecode level now just returns `Dataset[Object]`. This is really bad for Java users because they are losing type safety and also need to add a type cast every time they use range.

Talked to Jason Zaugg from Lightbend (Typesafe) who suggested the best approach is to return `Dataset[java.lang.Long]`. The downside is that when Scala users want to explicitly type a closure used on the dataset returned by range, they would need to use `java.lang.Long` instead of the Scala `Long`.

## How was this patch tested?
The signature change should be covered by existing unit tests and API tests. I also added a new test case in DatasetSuite for range.

Author: Reynold Xin <rxin@databricks.com>

Closes #11880 from rxin/SPARK-14063.