Improve robustness and usability of RealtimeIndexTask and some code cleanup in the Indexing Service

- Replace graceful/hard shutdown combo with simple exit
- Add RealtimeMetricsMonitor to RealtimeIndexTask
- Realtime plumber will start persisting segments shortly after finishJob is called, regardless of rejection policy
- Add "none" rejection policy
- RealtimeIndexTask: Allow configurable rejection policies
- IndexerCoordinatorResource: Log when remote task action fails