TEZ-4087: Shuffle: Fix shuffle cleanup to prevent thread leaks

[rbalamohan]

Merger threads are not killed in long running daemons. Work is in progress to mine the logs of long running daemons. As of now, all tasks are effectively cleaning up. However, from tez side, it would be good to have a feature where merger threads stop if the parent thread dies.

Changes:

1. Pass ShuffleScheduler detail to merger threads
2. Add attempt_id detail to merger threads. This would help in identifying thread details. As of now, only vertex name is populated which is not very useful.
3. In merger thread, periodically check if the parent thread is alive.

[jteagles]

TEZ-4087 was merged via apache gitbox. Closing.