DRILL-7418: MetadataDirectGroupScan improvements
1. Replaced files listing with selection root information to reduce query plan size in MetadataDirectGroupScan.
2. Fixed MetadataDirectGroupScan ser / de issues.
3. Added PlanMatcher to QueryBuilder for more convenient plan matching.
4. Re-written TestConvertCountToDirectScan to use ClusterTest.
5. Refactoring and code clean up.
