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* SQL: Ditch CalciteConnection layer and add DruidMeta, extension aggregators. Switched from CalciteConnection to Planner, bringing benefits: - CalciteConnection's JDBC interface no longer sits between the SQL server (HTTP/Avatica) and Druid's query layer. Instead, the SQL servers can use Druid Sequence objects directly, reducing overhead in the query return path. - Implemented our own Planner-based Avatica Meta, letting us control connection timeouts and connection / statement limits. The previous CalciteConnection-based implementation didn't have any limits or timeouts. - The Planner interface lets us override the operator table, opening up SQL language extensions. This patch includes two: APPROX_COUNT_DISTINCT in core, and a QUANTILE aggregator in the druid-histogram extension. Also: - Added INFORMATION_SCHEMA metadata schema. - Added tests for Unicode literals and escapes. * Verify statement is actually open before closing it. * More detailed INFORMATION_SCHEMA docs.