DRILL-6866: Upgrade to SqlLine 1.6.0
1. Changed SqlLine version to 1.6.0.
2. Overridden new getVersion method in DrillSqlLineApplication.
3. Set maxColumnWidth to 80 to avoid issue described in DRILL-6769.
4. Changed colorScheme to obsidian.
5. Output null value for varchar / char / boolean types as null instead of empty string.
6. Changed access modifier from package default to public for JDBC classes that implement external interfaces to avoid issues when calling methods from these classes using reflection.
closes #1556
