DRILL-7406: Update Calcite to 1.21.0
1. DRILL-7386 - added tests to TestHiveStructs.
2. DRILL-4527 - the DrillAvgVarianceConvertlet can't be removed without test failures.
3. DRILL-6215 - switched to prepared statement in JdbcRecordReader.
4. DRILL-6905 - added test into TestExampleQueries.
5. DRILL-7415 - Fixed jdbc show tables when 2 tables with same name are present in different schemas.
6. DRILL-7340 - Fixed jdbc filter pushdown when few jdbc datasources enabled.
7. Split SqlConverter into multiple source files.
8. Minor refactorings for jdbc and other places.
closes #1940
