[SPARK-19185][DSTREAM] Make Kafka consumer cache configurable

## What changes were proposed in this pull request?

Add a new property `spark.streaming.kafka.consumer.cache.enabled` that allows users to enable or disable the cache for Kafka consumers. This property can be especially handy in cases where issues like SPARK-19185 get hit, for which there isn't a solution committed yet. By default, the cache is still on, so this change doesn't change any out-of-box behavior.

## How was this patch tested?
Running unit tests

Author: Mark Grover <mark@apache.org>
Author: Mark Grover <grover.markgrover@gmail.com>

Closes #18234 from markgrover/spark-19185.