[SPARK-17841][STREAMING][KAFKA] drain commitQueue

## What changes were proposed in this pull request?

Actually drain commit queue rather than just iterating it.
iterator() on a concurrent linked queue won't remove items from the queue, poll() will.

## How was this patch tested?
Unit tests

Author: cody koeninger <cody@koeninger.org>

Closes #15407 from koeninger/SPARK-17841.