[SPARK-19968][SS] Use a cached instance of `KafkaProducer` instead of creating one every batch.

## What changes were proposed in this pull request?

In summary, cost of recreating a KafkaProducer for writing every batch is high as it starts a lot threads and make connections and then closes them. A KafkaProducer instance is promised to be thread safe in Kafka docs. Reuse of KafkaProducer instance while writing via multiple threads is encouraged.

Furthermore, I have performance improvement of 10x in latency, with this patch.

### These are times that addBatch took in ms. Without applying this patch
![with-out_patch](https://cloud.githubusercontent.com/assets/992952/23994612/a9de4a42-0a6b-11e7-9d5b-7ae18775bee4.png)
### These are times that addBatch took in ms. After applying this patch
![with_patch](https://cloud.githubusercontent.com/assets/992952/23994616/ad8c11ec-0a6b-11e7-8634-2266ebb5033f.png)

## How was this patch tested?
Running distributed benchmarks comparing runs with this patch and without it.
Added relevant unit tests.

Author: Prashant Sharma <prashsh1@in.ibm.com>

Closes #17308 from ScrapCodes/cached-kafka-producer.