[SPARK-13897][SQL] RelationalGroupedDataset and KeyValueGroupedDataset

## What changes were proposed in this pull request?
Previously, Dataset.groupBy returns a GroupedData, and Dataset.groupByKey returns a GroupedDataset. The naming is very similar, and unfortunately does not convey the real differences between the two.

Assume we are grouping by some keys (K). groupByKey is a key-value style group by, in which the schema of the returned dataset is a tuple of just two fields: key and value. groupBy, on the other hand, is a relational style group by, in which the schema of the returned dataset is flattened and contain |K| + |V| fields.

This pull request also removes the experimental tag from RelationalGroupedDataset. It has been with DataFrame since 1.3, and we have enough confidence now to stabilize it.

## How was this patch tested?
This is a rename to improve API understandability. Should be covered by all existing tests.

Author: Reynold Xin <rxin@databricks.com>

Closes #11841 from rxin/SPARK-13897.