[SPARK-14733] Allow custom timing control in microbenchmarks

## What changes were proposed in this pull request?

The current benchmark framework runs a code block for several iterations and reports statistics. However there is no way to exclude per-iteration setup time from the overall results. This PR adds a timer control object passed into the closure that can be used for this purpose.

## How was this patch tested?

Existing benchmark code. Also see https://github.com/apache/spark/pull/12490

Author: Eric Liang <ekl@databricks.com>

Closes #12502 from ericl/spark-14733.