Cassandra 19480 Additional task execution specific instrumentation of job stats

[arjunashok]

Changes

• Utilized spark listeners to publish job/task stats on job completion for reader and writer.
• Unified implementation of job end stats published for reader and writer
• Publishes task runtime stats
• Adds spark jobGroup based UUID to reader, similar to existing implementation in writer. This is used as the jobId to uniquely identify and potentially merge multiple stats published from the same job.

Testing

• Validated stats logged from unit tests and in-jvm-dtest integration tests for reader/writer
• Unit tests for Stats listener

[arjunashok]

Closing as duplicate of #51