[SPARK-21743][SQL] top-most limit should not cause memory leak

## What changes were proposed in this pull request?

For top-most limit, we will use a special operator to execute it: `CollectLimitExec`.

`CollectLimitExec` will retrieve `n`(which is the limit) rows from each partition of the child plan output, see https://github.com/apache/spark/blob/v2.2.0/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala#L311. It's very likely that we don't exhaust the child plan output.

This is fine when whole-stage-codegen is off, as child plan will release the resource via task completion listener. However, when whole-stage codegen is on, the resource can only be released if all output is consumed.

To fix this memory leak, one simple approach is, when `CollectLimitExec` retrieve `n` rows from child plan output, child plan output should only have `n` rows, then the output is exhausted and resource is released. This can be done by wrapping child plan with `LocalLimit`

## How was this patch tested?

a regression test

Author: Wenchen Fan <wenchen@databricks.com>

Closes #18955 from cloud-fan/leak.