Support CURRENT_CATALOG and CURRENT_SCHEMA

Support these keywords for querying the current catalog and schema. SQL reference: section 6.4

oracle
CREATE TABLE CURRENT_SCHEMA (col VARCHAR2(1)); – ok
SELECT CURRENT_SCHEMA FROM DUAL; – error, ORA-00904: "CURRENT_SCHEMA": invalid identifier
SELECT CURRENT_SCHEMA() FROM DUAL; – error, ORA-00904: "CURRENT_SCHEMA": invalid identifier

postgres
CREATE TABLE CURRENT_SCHEMA (col VARCHAR(1)); – error: syntax error at or near "CURRENT_SCHEMA"
SELECT CURRENT_SCHEMA; – ok, "public"
SELECT CURRENT_SCHEMA(); – ok, "public"

mysql
CREATE TABLE CURRENT_SCHEMA (col VARCHAR(1)); – ok
SELECT CURRENT_SCHEMA; – error, Unknown column 'CURRENT_SCHEMA' in 'field list'
SELECT CURRENT_SCHEMA(); – error, FUNCTION db_9_e28e6f.CURRENT_SCHEMA does not exist