[SPARK-18714][SQL] Add a simple time function to SparkSession

## What changes were proposed in this pull request?
Many Spark developers often want to test the runtime of some function in interactive debugging and testing. This patch adds a simple time function to SparkSession:

```
scala> spark.time { spark.range(1000).count() }
Time taken: 77 ms
res1: Long = 1000
```

## How was this patch tested?
I tested this interactively in spark-shell.

Author: Reynold Xin <rxin@databricks.com>

Closes #16140 from rxin/SPARK-18714.