DRILL-6324: Unnest - Initial Implementation
- Based on Flatten
- Implement unnestRecords in UnnestTemplate
- Remove unnecessary code inherited from Flatten/Project. Add schema change handling.
- Fix build failure after rebase since RecordBatchSizer used by UNNEST was relocated to a different package
- Add unit tests
- Handling of input row splitting across multiple batches. Also do not kill incoming in killIncoming.
- Schema change generated by Unnest
