implement timestamp aggregation in DBs themselves

src/metabase/driver.clj

src/metabase/driver/context.clj

src/metabase/driver/generic_sql.clj

src/metabase/driver/generic_sql/query_processor.clj

src/metabase/driver/generic_sql/util.clj

src/metabase/driver/h2.clj

src/metabase/driver/postgres.clj

src/metabase/driver/query_processor.clj

test/metabase/driver/query_processor_test.clj

test/metabase/test/data/dataset_definitions.clj